Punctuation marks are important for recognizing the end of a statement and or drawing some out with an ellipsis. If you do not put them at the end of your writing/sentence, unless using a question mark(?) or exclamation point(!), then the reader/grader will not be able to know where your sentence ends and the new one begins, therefor rendering it a run on sentence.
Answer:
When the children grow up
I describe love as family
D.) to emphasize if "you" are coming to the tree.
