If the concept is a truck, the prototype may be:

Question

a) a Ford pickup
b) bulldozer
c) an eighteen-wheeler
d) personnel carrier

I think c

Answer 1

http://www.thefreedictionary.com/prototype
Read definition 1 -- then re-think.

Answer 2

I'm thinking this may be where me being from the UK is giving me a disadvantage in the USA. I asked others what they think a truck is so I'm now thinking a?

Answer 3

Yes.