Select the most appropriate option to fill in the blank.

You will find it difficult _______ but later you will get used to wearing a mask.

at first

he most appropriate option to fill in the blank is at first.

• On first is not a grammatically correct phrase.
• Firstly is an adverb that means "in the first place." It is not appropriate to use here because it does not make sense to say "you will find it difficult in the first place to wear a mask."
• At the first is not grammatically correct.