Which of the following acids has the weakest conjugate base?

Question

a....4.2E-09
b...9.2E-03
c....6.3E-06
d....1.2E-01

Answer 1

You wrote only the answers. But to answer the unasked question, here is how you do it.
Kb = Kw/Ka. Kw = 1E-14. Plug in Ka and solve for Kb, then arrange either strong to weak or weak to strong.