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].

explanation:

let the different atoms be x and y. each with four protons and same mass will be represented as follows.

mass number is number of protons + number of neutrons.

            and  

since, it is given that one has a negative charge while the other is neutral. therefore, there will be increase in number of protons and the atom will be represented as follows.

according to periodic table, mass number of beryllium is 9 and it has atomic number 4 so, the atoms will be and .  

the protons and neutrons will be located inside the nucleus whereas the electrons will be revolving around the nucleus.