The correct answer is:
Are you free on Saturday Afternoon?
In, On and At are prepositions of place. We use In to show that something is inside or surrounded by something else. For example:
- The children are in the classroom.
- The dinner is in the oven.
- Are there chocolates in the box?
- I live in Budapest.
On is used to indicate that something is touching the surface of an object or to talk about days of the week, calendar dates and special dates such as birthdays. For example:
- The book is on the sofa.
- The lecture is on Monday.
- My Birthday is on the seventh of January.
- We go to Hawaii on our wedding anniversary every year.
At is used to point out a specific location, time or position. For example:
- They travel at the weekend.
- The meeting is at midday.
- At night, it is very cold.
- The match will start at 3 P.M.
This is why we use On in the sentence from the quiz as On is used to talk about days of the week.