A confused Question!

[BHoury]

hello
I've taken an English test, and there was a question which confused me, and no one of my teachers here could convince me why I answered it wrongly. here is the question:

Microsoft has offices around the world, but they are .......................... in Seattle.
a) base b) basic c) based d) basically

They said that the correct answer is (c) , but I think that (d) can be used also.
what do u think about it ? and why?

and thank you very much.

Bashar

[mykwyner]

Basically is an adverb and must be used to describe a verb, an adjective, or another adverb.

He is basically kind and considerate.

Based, in this case, means "the location of their home base."