How does connecting a capacitor across a DC power supply with varying voltage produce a constant voltage?
I agree with pjc50 and want to stress the severity of the problem. The exam test #16 says:

"Figure 1 shows the output from the terminals of a power supply labelled d.c. (direct current)." 

This looks OK and is well recognizable to a seasoned engineer as an output from a bridge rectifier, with all implication of asymmetrical diodes and such. But how is a schoolchild supposed to know this? The "educators" failed to to tell. 

I noticed that the concept of realistic source impedance is not taught/mentioned. Although the question (a) might be reasonably formulated (but still with ambiguity), without information on the nature of source impedance all following tasks are meaningless and confusing. 

This exam text seems to be in circulation for a couple of years. I can imagine how much frustration and emotional distress was inflicted by this faulty test on young and especially mathematically-inclined minds.