My question is:
A common mushroom (that you would purchase in a grocery store) mostly consists of tissue that is:
c) in the heterokaryon phase
d) in the phylym Zygomycetes
e) in the same phylum as angiosperms
That is true: the heterokaryon stage is a special case, if you want to call it that. I see nothing wrong with calling a keterokaryon diploid, but as the majority of books make the distinction, i accept it. However, the keterokaryon is a transient stage(pardon the word, some fungi retain this stage for many years) in the sexual reproduction of a fungus.
The fungus body, the mycelium, is haplod. Here is a nice picture of the life cycle of a fungus. From Biology 7th edition, Campbell and Reece