This is particularly pernicious, because it's a trick question, too. On linux, malloc always returns, it will never return NULL. Even if you ask for 4 petabytes of memory on a 128mb system, malloc will hand you back a valid pointer for the memory.
If I were asked that, I would say something like "malloc attempts to allocate some memory on the heap and return a pointer to it." If pressed for more details, I would say it depends on the allocator and that we should look at the documentation for it to find out.
If that was an unacceptable answer, then I'd consider the interview a waste of time.
