Those customers are ____ the most demanding we've ever had.
-
so far
-
from far
-
since far
-
by far
D
Correct answer
Explanation
The correct phrase is 'by far' when making a superlative comparison or emphasizing something is the best/worst. This is an idiomatic expression meaning 'by a great margin' or 'clearly'. Options A, B, and C use incorrect prepositions that don't form the standard comparative intensifier phrase.