This question is testing MODIFIERS and PARALLELISM/COMPARISONS.

(A) When working with overseas clients, an understanding of cultural norms is at least as important as grasping the pivotal business issues for the global manager.
- "when working..." is an opening modifier. It must modify the subject that comes after it. Here, the subject is "an understanding", implying that the understanding is working with clients.
- the placement of "for the manager" is ambiguous - it could imply that the issues are for the manager.

(B) When they work with overseas clients, understanding cultural norms is at least of equal importance to the global manager as grasping the pivotal business issues.
- similar opening modifier issue to A. If we start with "when they work...", the subject must be the antecedent of "they." It should be "managers," not "understanding."
- "of equal importance as" is not idiomatic.

(C) For global managers working with overseas clients, understanding cultural norms is at least as important as grasping the pivotal business issues.
Correct!

(D) For global managers working with overseas clients, an understanding of cultural norms is at least as important to them as that they grasp the pivotal business issues.
- We need a parallel comparison. "an understanding... is as important... as that they grasp" is not parallel - it's comparing a noun to a clause.
- "to them" is redundant after "for managers."

(E) Global managers working with overseas clients find an understanding of cultural norms to be equally important as grasping the pivotal business issues.
- "equally important as" is not idiomatic. We should say "as important as."
- "an understanding" is not parallel to "grasping." It would have to say "the grasping of."

The answer is C.