Which of the following is an important rule regarding colon usage?
a. a colon can't be used in a question.
b. a colon only introduces a list.
c. a colon points to an idea in an earlier sentence.
d. the words coming before the colon must form a complete sentence.

B. A colon only introduces a list. 

Colons denote "that is to say" or "here's what I mean." They make sentences’ first parts not important rather the sentence’s second part contains the important information. Here are some of the rules you could use for colons:

*Colons are uses upon introducing items. 

*Do not use capital letters after a colon except if it’s a proper noun. 

*Do not use colons after a verb or preposition. 

*For independent clauses, especially when the second sentence explains the paragraph’s meaning, the use of colon is preferred. 

*After a colon: capitalize the first letter of the first word if it is a complete quotation.