Explanation:

- Option 1: on first

- "On first" is not a commonly used phrase for indicating the beginning of something. It sounds awkward in this context.

- Option 2: at first

- "At first" is the correct phrase. It means initially or in the beginning. It fits well here, describing initial difficulty followed by getting used to wearing a mask.

- Option 3: firstly

- "Firstly" is often used in lists to indicate the first point or reason. It's not suitable for describing the initial experience.

- Option 4: at the first

- "At the first" is not a standard expression in English. It's incorrect in this context.