Which of the following is a buffer system? A. NaCl and NaNO3
B. H2CO3 and KHCO3
C. HCl and NaOH
D. H2O and HCl
E. NaCl and NaOH

B. H2CO3 and KHCO3

Explanation:

The buffer system is a system in which the solution deny for change in the ph level i.e. at the time when the acid is added to it. It can be made either in a weak acid and salt or can be a weak base and its salt. Now when the acid is added to a buffer so the ratio is not impacted due to which it does not change sufficient for pH level

As per the given options, the third one is correct as the H2CO3 denotes a weak acid and the KHCO3 represents the salt. And these two solution does not create any change in the level of pH

Therefore the option B is correct