The single most important ingredient in a good beer is high-quality

a. malt.
b. yeast.
c. hops.
d. water.

The single most important ingredient in a good beer is high quality WATER. The correct option is D.
The quantity of water in beer is more than 90%. High quality water is the most important ingredient that is required in beer making process. 
After water comes barley malt, hops and then yeast respectively.

the main ingredient is Malt

Yeast is for the fermentation

Hops mainly for the taste

Water is 95% of this beveragebut is it considered as a main ingredient...in quantity yes, but it doesn't make a beer !!!

hope I helped

The answer is either 3 or 4