b. but why ask here? Do you not have a tutor? In any event, what is on the answer sheet?

b. but why ask here? Do you not have a tutor? In any event, what is on the answer sheet?

Don’t start the course till April found some mock exams online thought I would make a start, I thought the answer was b they put it down as d 

Good for you!

That diagram is a little old-fashioned. There, the supply is TN-S, which is shown by the clamp around the (lead-sheathed) service cable, which connected to the MET by the earthing conductor (A).

Nowadays, services tend to be in plastic pipe so there is no requirement for main protective bonding so B and C would not be required. The supply is likely to be TN-C-S (PME) so the earthing conductor comes out of the service head. There is no requirement for the MET as shown and the earthing conductor goes straight to the distribution board. The MET is now the earth terminal bar in the DB and they typically have provision for bonding conductors.

If you put that all together, the earthing conductor would be A and D so I can see how they have made their mistake.

If you have a traditional MET by the service head, and a cable from there to the DB, I am not sure what cable D is called. It does not match the definition for a circuit protective conductor, an earthing conductor, or a bonding conductor. I think that you could simply call it a "main protective conductor", but many people would probably regard it as (part of) the earthing conductor.

I don't want to confuse you, but sometimes when there is a submain between buildings, its protective conductor joins two terminal strips of the same type as a traditional MET. You can have only one MET, but the one in the second building serves the same purpose.  As Mike says, it could be known as a "marshalling terminal", but I prefer "building earth terminal".

HTH.