B.

The prompt is too ambiguous to answer this properly. If we're allowed to "measure out" the paint in the sense that we can take 3/4 of a gallon, then B is the answer: we buy 1 gallon of white and 1.5 gallons of black, then mix .75 gallons of white with 1.25 gallons of black for 2 gallons of the mixture.

If we AREN'T allowed to measure smaller portions of the gallons -- i.e. we have to just throw everything we buy in a paint mixer -- then the answer is E, since we'd need 1.5 gallons of white and 2.5 gallons of black.

E feels too easy, though, so I'd assume the OA (and the intended answer) is B.

I think Final Answer is B.

Didn't get how other reach at E ?

Question clearly asks for "what is the least amount of paint, in gallons "

that must be purchased in order to measure out the portions needed for the mixture" --

which means portions can be measured.

1. Taking E as answer will yield to 4 litres of Grey paint. -- But we need 2 litres of Grey paint. If There is no way to measure 2 litres out of 4 litres then E can't be the answer.