I got this one wrong, but I still haven't been able to work out where it says in the regs. The only bit about Cable Couples in there (that I found) was very vague.
But generally speaking, I would have thought the socket (female) side would always be on the supply side so that there was never a dangling live connector?
Or are they using cable coupler in a sense I'm not familiar with? (I'm thinking supersized extension cable used in marine/mining etc)

28. Cable couplers should be arranged so that the plug of the coupler is connected to
a. the load side
b. the supply --> CORRECT ANSWER
c. the terminals of the consumer unit
d. a splitter box
YOUR ANSWER: a