Question

Which sentence uses a colon correctly?

Answer 1

Answer: C) Projects may be dropped off at the following locations: Room 16, Room 78, or Room 3.

Explanation: A colon is a punctuation mark usually used to introduce a list of elements or an explanation. A colon should never separate a verb from its object (like its happenning in option A, separating the verb "unloaded" from its objects "the canned food" "vegetables", etc), it also shouldn't be used to separate a preposition from its object (like in option B and D, separating "to" and "from" from their objects). So the correct answer is option C, where the colon is introducing a list of elements.

Answer 2

The sentence, "Projects may be dropped off at the following locations: Room 16, Room 78, or Room 3." uses the colon correctly. 
The definition of Colon is, "a punctuation mark (:) used to precede a list of items, a quotation, or an expansion or explanation."