Which of the following substances best conducts electricity in an aqueous solution?

a. H2O b. NH3 c. CH3OH d. CaCl2

The correct answer is definitely D. CaCl2

H2O can't be a productor just by itself as it need electrolytes.

CH3OH is non-polar one, so throw it.

You need something form ionic compound range, so the last one is the only suitable answer.