Select the most appropriate option to fill in the blank.

My son turned a year older on his birthday recently ______ March 14.

on

The most appropriate option to fill in the blank is on.

The word "on" is used to indicate a specific day or date. This is the most appropriate word because it accurately describes how the son's birthday was on March 14.

The other options are not as appropriate.

• At can be used to indicate a specific time or place. This does not make sense in the context of the sentence, because the son's birthday is not a specific time or place.
• Over can be used to indicate a period of time. This does not make sense in the context of the sentence, because the son's birthday is not a period of time.
• In can be used to indicate a general period of time. This does not make sense in the context of the sentence, because the son's birthday is a specific date.