"This is a difficult question."
Translation:यह एक मुश्किल सवाल है।
I didn't include the "एक" in my answer (i.e. यह मुश्किल सवाल है), so was marked incorrect. However, isn't the singular nature of the difficult question obvious without it, because यह and है are singular?
A bit of a shot in the dark, but I think यह मुश्किल सवाल है। could also be understood to mean "There is this difficult question", as in:
"तुम क्या सोच रहे हो? कुछ problem है?"
"यह मुश्किल सवाल है।"
What are you thinking? Is there some problem?
There is this difficult question (that must be addressed).
That seems a plausible situation. But perhaps that is more nuanced than the question writers intended.