I can’t really see the picture
The answer is
Yet.
A coordinated conjuction is a word that joins or coordinates two senteces together, in this case the word yet is conectint he was but a child, and giving meaning to the whole sentece by conecting to he did so much for others, if you read it without the yet, the sentence looses force in the statement.
D because all the others are much more formal but with D it’s contrasted.
Hope this helped :)
