HELP ME ANSWER THESE PLEASE ! biochemistry

Hey so i would appreciate any guidance on these questions that I found from an old textbook. I've attempted but just can't seem to answer them. Thank you.

1) What is the actual function of a buffer system such as acetate / acetic acid?

Select one:
a. To maintain the solution at neutral pH
b. To maintain the solution at the pKa
c. To ensure that the solution has no free H+ or OH- within a specific range
d. To maintain a constant pH in the event the concentration of an acid or alkali increases
e. To keep the solution within one pH unit of the pKa

2) In what area of the titration curve for carbonic acid (H2CO3) is the molecule fully deprotonated?

Select one:
a. Far right
b. Far left
c. In the area about 1 pH unit higher than the pKa
d. At the second pKa value, given there are two deprotonations
e. At the center, at the pKa

3) What is the pH of a 0.1 mM solution of hydrochloric acid at 37°C?
Answer:

4) What is the concentration of OH- in a solution with a H+ concentration of 1.3 X 10-4 M?
Answer: