Question: #1: I believe answers B, C and D all equally express a possibility. #2: Answer B also offers advice and a recommendation. #3: Answer C expresses a recommendation, and would thus also be grammatically correct. I would disagree with the implication that "could" applies to only to "past activities." The sentence "I could go to the beach tomorrow" is of course perfectly correct. #5: If "at work" is the only place the phone could have been left, then yes, "must" is correct -- otherwise, answers B, C or D. I think I'll end here. To me, it seems like most of the sentences were phrased in a manner that allows for more than one correct answer. "Should" and "ought to" are virtually synonymous in suggesting an obligation; "may" and "might" and "could" all suggest a possibility.

Loop moo