The correct option is C is
It is to be kept in mind that Either, Neither, Each, Everyone, Many a, must be followed by a singular verb.
'Neither pen' means none of the pen. So, we are considering the unavailability of each individual pen. By the general convention, a singular noun takes a singular verb.
Hence, Option C 'is' is correct and Option B 'are' is incorrect.
Options A and D are incorrect as they are completely irrelevant to the context.