GMAT Sentence Correction (SC) | A proposed law would require people to bring photo ID

A proposed law would require people to bring photo ID topolling places when they vote to curb potential fraud.

A.places when they vote to curb potential fraud
B.places when they vote, to curb potential fraud
C.places, when they vote to curb potential fraud
D.places, when they vote, to curb potential fraud
E.places, to curb potential fraud by voting

Can some one tell how to decide placement of command and the subject , which is modified by the "to curb" phrase?
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