Colons are generally used to separate independent clauses which relate to each other. (Independent clauses always express a complete thought and can stand on their own.) The first clause before a colon essentially prompts the next part of the sentence.
Example: This very sentence I'm writing.
Let's look at each sentence.
A. Instead of complaining, Mark Covington decided to spend his time: working to save his neighborhood.
Both of these clauses are dependent clauses; they do not express complete thoughts. (In the first one, what is Mark spending his time doing? In the second clause, who is working to save the neighborhood?) These clauses cannot stand on their own, and the colon is completely unnecessary in this sentence.
The correct version of this sentence: <em>Instead of complaining, Mark Covington decided to spend his time working to save his neighborhood.</em>
B. Mark Covington could think of one way to rebuild his neighborhood: establish a community garden on an abandoned lot.
This sentence is comprised of two independent clauses separated by a colon, and the second clause further elaborates on the idea of the first clause. The colon is used correctly in this sentence.
C. Now there is a thriving community center and garden: where the vacant lot used to be.
This sentence is similar to sentence A, where both clauses on their own don't express a complete thought. The colon is misused here.
The correct version of this sentence: <em>Now there is a thriving community center and garden where the vacant lot used to be.</em>
D. Like many other Detroit natives, Mark Covington experienced: job loss as a result of an economic downturn.
Same issue as sentences A and C: the two clauses on their own don't make complete sense as they lack either an object or a subject.
The correct version of this sentence: <em>Like many other Detriot natives, Mark Covington experienced job loss as a result of an economic downturn.</em>
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B. Mark Covington could think of one way to rebuild his neighborhood: establish a community garden on an abandoned lot.
