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Solutions
The possible pictorial representation of relationship is given below

Conclusions:
I. No key is a pen - False as all keys are pens.
II. No pen is a key – False as all keys are pens so some pens are keys also.
III. Some pens are keys - True as all keys are pens so some pens are keys also.
IV. Some keys are fishes – False as no fish is pen and all keys are pens so no keys are fishes.
So, only conclusion III follows.
Hence, option B is the correct answer.