Which group is a complete sentence?
a. started his homework late sunday night.
b. the exciting end of he baseball game.
c. she did not understand what happened.
d. two energetic women walking in the park.

C.

The others are not complete thoughts, they are dependent clauses.
The exciting end of a baseball game (what about it?)
Two energetic women walking in the park... came across a pond. (That's better)
Started his homework late Sunday night.. and as a consequence he was late to school on Monday. Now it sounds more complete