mjhjmtu

I'll go for

8

Because:

All other sentences contain words with a letter showing up twice in sequence, e.g. "look", "across" and "tree".

Is it:

5. It's the only sentence with all the vowels (aeiou) in it.

I have to say

6

because

it's actually two full sentences conjoined by a semicolon, rather than separated by a period. As such, it is the only one with more than one independent clause (number 5 has two phrases, but the first one is subordinate).

Perhaps it's

1

because

It's the only one whose contents would have been incomprehensible before "modern times". Let's define that as pre-Victorian. In defense of this answer, it turns out pie has been around for 2500 years(!), and the band reference, even if it is a group of people and not a strip of material, could be understood as early as the 15th century.

and

The poster winked repeatedly while making reference to old timey language, so maybe that was a clue.

and

It even says it in the title, for goodness's sake.

Is it

4?

Because

It is the only sentence that is a question mark.

Possibly

5

because

It's the only sentence where an odd number--specifically, "one"--appears, suggesting a play on the puzzle title, "Odd One Out". 5 also happens to be the first digit of the 5-digit number representing the total number of answers this puzzle will be spammed with if you don't narrow the parameters or provide a clue. *hint hint*

My guess is that it's

6

because

it is the only sentence written in future tense, the rest are present or past tense.

Just a guess, but worth a shot?

I am not to sure about this, but I think it is:

2

Because:

all other sentences have an odd number of syllables. (yes for 10 if you read "trotted" in one syllable)

I feel that it could be

4

because

It is the only sentence that is not saying something directly. It is asking a question, not stating something.

Another thought I had was that it could be

1

because

It uses American varieties of words (airplane). No others use them.

The odd one out is

7

because

nobody picked 7 yet it is not gender-neutral.

I think it's

2

Because

You can't walk across water.

Is it

5

Because

As it is the only statement with an infinitive. I think it might have two of them actually: to-infinitive "To eat" and a zero-infinitive "must (first) bake"

Is it

2

Because

There is no clear identification who it is referring to; Airplane, They(?), Tree, Painting, Apple pie, I, He, You, You, Horse