This data sufficiency question asks the following: "What fractional part of the total surface area of cube C is red?"

(1) Each of 3 faces of C is exactly 1/2 red.
(2) Each of 3 faces of C is entirely white.

The correct answer is C, because you need both information to conclusively answer the question. I chose answer A because I felt that the first statement was sufficient to answer the question about the fraction of cube C's surface area being red. Is my error an example of a technique mentioned in the Data Sufficiency chapter

It's really just a GMAT trick.
They tell you that 3 of the faces are exactly 1/2 red.
However, we don't know anything about the other three faces, and so we can't answer the question.

Veritas Prep