As given in statement:
[H+] = 10^-12
Formula for pH:
pH = -log [H+]
Putting the value of H+ ion in formula:
pH = -log ( 10^-12)
pH = 12.0
pH 0-6 (Acidic)
pH 7 (neutral)
pH 8-14 (basic)
A solution with pH 12.0 is basic.
So according to above explanation
a. The solution is basic in nature, is the correct answer.