A colon is mostly used to show that you are going to make a list of things after the comma. So, having that in mind, the correct answer is the first option - I edited the article and found: comma splices, sentence fragments, and misplaced modifiers.
You listed three things here, therefore you have to put a colon. If you are going to mention only one thing, like in the other examples, a colon is not needed.
Answer:
Option C.
Explanation:
He wanted to guard the door, so he did it quickly
I assume its that they are poor
Answer:
D
Explanation:
A-C would make great details in the report, but they do not explain his opinion on the topic (the VALUE of field trips).
Answer:
sphinx-like smile means a mysterious smile