I think it is C, because if I'm right present principal is something that is happening presently? If so, C would be the answer because they "are doing" the exercises right now in context.
Answer:
The second sentence is correct
Since a participle takes the form: verb + -ing, the only word in this sentence which has such a form is B, 'challenging'. 'Puzzle' is a noun, 'crossword' is an adjective, 'created' is a verb.
Answer:
He ran on a few steps, stopped, and looked back. She had not moved
<em>sorry is late answer</em>
