Let's see. The sentence is: "What is the longest word in this sentence?" I need to find out which word is the longest in this sentence.

First, I should probably list out all the words in the sentence. Let's break it down:

1. What

2. is

3. the

4. longest

5. word

6. in

7. this

8. sentence

Now, I need to count the number of letters in each word.

• What: 4 letters

• is: 2 letters

• the: 3 letters

• longest: 7 letters

• word: 4 letters

• in: 2 letters

• this: 4 letters

• sentence: 8 letters

Okay, so "sentence" has 8 letters, and "longest" has 7 letters. So "sentence" is longer.

Wait, but is "sentence" the longest? Let me check again.

What: 4

is: 2

the: 3

longest: 7

word: 4

in: 2

this: 4

sentence: 8

Yes, "sentence" has the most letters with 8 letters.

But wait, is there any word with more than 8 letters? Let's make sure.

Looking back at the list:

• What: 4

• is: 2

• the: 3

• longest: 7

• word: 4

• in: 2

• this: 4

• sentence: 8

No, "sentence" is the longest word in this sentence.

But now I'm thinking, maybe there's a word that's longer that I'm missing. Maybe there's a word within "sentence" that's longer, like if "sentence" contained another word within it.

Wait, no. Each word is separate. I think I listed all the words correctly.

Alternatively, maybe there are multiple words with the same length, but in this case, "sentence" is the longest.

Wait, perhaps I should consider punctuation. Is there any punctuation that might be considered part of a word?

The sentence ends with a question mark, so "sentence?" would be 9 letters if the question mark is included. But typically, punctuation is not considered part of the word. So, "sentence" is still 8 letters.

Alright, so "sentence" is the longest word with 8 letters.

But just to be thorough, maybe there's a word that I missed or miscounted.

Let me count again:

• What: W-H-A-T, 4 letters

• is: I-S, 2 letters

• the: T-H-E, 3 letters

• longest: L-O-N-G-E-S-T, 7 letters

• word: W-O-R-D, 4 letters

• in: I-N, 2 letters

• this: T-H-I-S, 4 letters

• sentence: S-E-N-T-E-N-C-E, 8 letters

Yes, definitely 8 letters.

So, the longest word is "sentence."

But now I'm thinking, maybe there's a word with a hyphen or something that I need to consider. Like "well-known" would be two words, well and known, but without a hyphen, it's one word.

But in this sentence, there are no hyphenated words.

Alternatively, maybe there are contractions, but there are no contractions in this sentence.

So, "sentence" is clearly the longest word.

But to make sure, perhaps I can measure the length programmatically.

If I were to write a simple program to find the longest word in a sentence, I would split the sentence into words and then find the word with the maximum length.

In Python, for example, I could do something like:

sentence = "What is the longest word in this sentence?"

words = sentence.split()

max_word = max(words, key=len)

print(max_word)

That would output "sentence?" but since punctuation is not part of the word, I should remove punctuation.

So, perhaps:

import string

sentence = "What is the longest word in this sentence?"

words = sentence.translate(str.maketrans('', '', string.punctuation)).split()

max_word = max(words, key=len)

print(max_word)

This would output "sentence" without the question mark.

So, yes, "sentence" is the longest word with 8 letters.

But now, out of curiosity, I wonder if there are any words with more than 8 letters in this sentence. Clearly not, but maybe in a different sentence.

Alternatively, perhaps there's a way to make "sentence" longer by considering it with punctuation, but as established, punctuation is not part of the word.

Wait, maybe there are words with apostrophes, like "it's," but in this sentence, there are no such words.

So, in conclusion, "sentence" is the longest word in the sentence "What is the longest word in this sentence?" with 8 letters.

Final Answer

[ \boxed{\text{sentence}} ]