The correct option is A: When training a dog, the trainer must remember two things: the dog does not comprehend the trainer's language, and the dog cannot read the trainer's mind.
A colon may be used between independent clauses when the second sentence explains, illustrates, paraphrases, or expands on the first sentence. The colon is used to introduce a list of items. A colon is also used before a quotation, and sometimes before direct speech.
In this case two items are introduced after the use of colon which are what the trainer must remember.
Answer: The student's homework was too difficult to be completed correctly.
Explanation: Lack of apostrophe in students. And randomly adding many didn't make sense to me.