Voters may not think much of parties, but large majorities still admit to party preferences and use parties to guide their voting decisions because:.
a. there is a residual irrational loyalty to parties that has outlived the usefulness of the party label.
b. party labels still provide the cheap, shorthand cue so useful to less-informed voters.
c. third-party candidates have unclear messages and ill-defined goals.
d. they have no real choice but to do so.

B. because party labels still provide the cheap, shorthand cue so useful to rationally ignorant voters

Explanation:

The reason is that, for most ignorant voters who have little or no idea about the candidates vying for electoral position, they make use of party labels; which is often times considered as the main ideology or objective of the party candidates.

To them, it serves as a clue to what they will be getting from the candidates if elected, hence, majority of voters still use parties to guide their voting decision.

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