n his first day of working at the junior baseball camp, Sam taught the children how to catch a fly ball, throwing a fastball, and, in an effort to improve their scoring, how to steal a base.

The best way to write the underlined part of the sentence is:

D. how to throw a fastball, and, in effort to improve their scoring

This option maintains parallel structure and clarity in the list of activities Sam taught the children. However, it should be noted that it contains a small grammatical error, as it should say "in an effort to improve their scoring." Based on the provided choices and the intent to maintain the original meaning while correcting for fluidity, "D" is still the closest to being correct.