Answer:
option B, of humans
Explanation:
Of human is correct because in the structure of sentence blood word is used preceding to the organism to which it belongs. Thus , a connecting word is required to establish a relationship between blood and human. Therefore "of " is used before blood to connect it with the organism to which it belongs.
If blood has not been used earlier in the sentence , option A would have been correct.
considering all explanations above option B is the right choice of answer.
