Answer: challenging problems
Explanation:
Whatever completes the text would have be something that relates to the problems that are to be solved as this is the subject of the sentence and it would have to be in the negatives due to the use of the word "unfortunately".
The best option is therefore the above because too many challenging problems relates to the subject of problems and is not a good thing.
A is the answer I have read the story but I don't remember that much
