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].

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definition

                    entropy is a measure of the randomness or the extent of disorder of a chemical process.

                      enthalpy is a measure of the heat change of a reaction occurring at a constant pressure.

measurement units:

                        entropy is measured in joule per kelvin (jk⁻¹)   whereas enthalpy is measured in joule per mole (jmol⁻¹)

requirements:

                            entropy has no requirements or limits, and its change is measured by the division between the heat change of the chemical process and the temperature.

                              in contrast, enthalpy can only be related to the heat change of the reaction under standard conditions.