a) pH decreases slightly. Addition of an acid always decreases pH.
b) It's a weak acid, so...barely anything.
c) jack ****. You're diluting the entire solution, so the acid molarity decr in equal proportions to the base molarity decr.
You can't say with confidence that a weak acid or a weak base will do NOTHING to the pH of a buffer without other information. How weak of a buffer or strong of a buffer is it? How much weak acid is added?
Likewise, you need to know concentrations and amounts to say the decrease in pH is "very slight" upon the addition of a strong acid.
Know that a strong acid and a strong base will cause a decrease and an increase in pH respectively, while a weak acid and a weak base will cause a decrease/increase in pH based on how the pKa/b of the weak acid/base relates to the pH of the buffer. Also know that water will generally not change the pH of the buffer due to the reason mentioned above.