I need help understanding buffer solutions. Which of these are buffer solutions?
a. 0.1 M HCl and 0.1 M NaCl
b. 0.1 M formic acid and 0.1 M sodium formate
c. 0.1 M HCl and 0.2 M HF
d. 0.3 M HNO2 and 0.009 M NaNO2
e. 0.2 M NH3 and 0.15 M NH4Cl
f. 0.3 M Acetic acid and 0.003 M sodium acetate.

I'm assuming a and c isn't, because those contains strong acids. b isn't because the amount is not significant enough? but i'm unsure about d,e, and f. need help understanding.

Question

I need help understanding buffer solutions. Which of these are buffer solutions?
a. 0.1 M HCl and 0.1 M NaCl
b. 0.1 M formic acid and 0.1 M sodium formate
c. 0.1 M HCl and 0.2 M HF
d. 0.3 M HNO2 and 0.009 M NaNO2
e. 0.2 M NH3 and 0.15 M NH4Cl
f. 0.3 M Acetic acid and 0.003 M sodium acetate.

I'm assuming a and c isn't, because those contains strong acids. b isn't because the amount is not significant enough? but i'm unsure about d,e, and f.  need help understanding.