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.

productive most commonly means: achieving or producing a significant amount or result.

explanation:

productive can also mean:

producing or giving rise torelating to or engaged in the production of goods, crops, or other commodities.

these depend on how the word productive is used. most likely productive will mean   achieving or producing a significant amount or result.