Which of the following is true?
a. an effective buffer has a [base]/[acid] ratio in the range of 10 - 100.
b. a buffer is most resistant to ph change when [acid] = [conjugate base].
c. an effective buffer has very small absolute concentrations of acid and conjugate base.

B. A buffer is most resistant to pH change when [acid] = [conjugate base].

Explanation:

Considering the Henderson–Hasselbalch equation as:

If the concentration of the acid and the conjugate base becomes equal, then the pH becomes equals to pKa which is a constant.

At this concentration , further addition of anything (acid or base) in any amount will not change the pH of the buffer as it has become constant.

So, a buffer is most resistant to pH change when [acid] = [conjugate base].