1. He is doing a sit-up.
2. He is doing sit-ups.
3. He can do up 100 sit-ups.
(Are they all grammatical?)
4. Am I favorite?
5. Am I his favorite?
6. Am I favorite of him?
7. Am I favorable to him?
(Which one is grammatical?)
1 and 2 are fine. In 3, you need to add "to" after "up".
4 and 6 are incomplete.
5 is the best.
7 has a different meaning.
1. He is doing a sit-up. ✔️
2. He is doing sit-ups. ✔️
3. He can do up to 100 sit-ups. ✔️
All three sentences are grammatically correct.
4. Am I favorite? ❌
5. Am I his favorite? ✔️
6. Am I favorite of him? ❌
7. Am I favorable to him? ✔️
Out of the four options, sentences 5 and 7 are grammatically correct.
Explanation for Sentence #4:
When asking if you are someone's favorite, you should use the pronoun "his" before "favorite" to indicate possession. So, "Am I favorite?" should be revised to "Am I his favorite?".
Explanation for Sentence #6:
The phrase "favorite of him" is not grammatically correct. Instead, you should use the possessive form of the pronoun "his" before "favorite". Therefore, "Am I favorite of him?" should be revised to "Am I his favorite?".
In summary, sentence #5 ("Am I his favorite?") is the correct grammatical form.