I think this would be a or d
The most important thing to remember about colons is that you only use them after statements that are complete sentences.
Answer:
Billy is the complete subject of this sentence because he is what the sentence is about.
Explanation:
Billy is the complete subject of this sentence because he is what the sentence is about.
Writing on a computer is faster than writing sentences out on paper, and it also allows you to correct mistakes much easier than on paper. Hope this helped!
-TTL