1 – Correct
I always remember it like this: Ammonia has fewer letters (in the word Ammonia itself) than Ammonium.
So Ammonia is NH₃ (1 less Hydrogen since it has fewer letters) and Ammonium is NH₄.
But everyone has different ways of remembering things!
2 – Wrong
Ammonia in water is a weak alkali. So the pH would be slightly higher than 7.
3 – Correct
Ammonia has a molecular structure.
4 – Wrong
Again, you must come up with things which help you remember concepts like these.
I remember it like this:
Acid has four letters and blue has four letters. So an acid turns blue litmus red.
So Alkali must be the other way round. Alkali turns red litmus blue.
5 – Wrong
Ammonia is a gas at room temperature.
Nitrogen atom covalently bonds to 3 Hydrogen atoms to form Ammonia.
Statements 1, 3 and 6 are correct. So D is the answer.