Which is the best meaning for average as it is used in this sentence?
while it doesn’t look much different than the average vitamin capsule, the work it does inside the body, is truly amazing.
a) definition 1
b) definition 2
c) definition 3
d) definition 4

4. Any medial estimate or general statement derived from a comparison of diverse specific cases; a medium or usual size, quantity, quality, rate, etc. The average of sensations.

Explanation:

In the sentence, the word average is referring to size, color, shape or any other trait that would indicate how the capsule looks. The speaker says that it looks similar to any other vitamin capsule. Since it does such awesome things for the human body, one might think the capsule should also look awesome, but the speaker indicates that it doesn't.

the definitions are not listed here, but i should say that the most likely definition for "average" in this sentence is something along the lines of "normal, seen often".

buy a huge library full of all the books ever written

well if you have a lion for example it will kill you and that's why you shouldn't have and exotic animal