Gerrymandering can thwart the purpose of single-member district elections because it allows?

The simple answer is that the districts could be drawn to favor one party.

Gerrymandering in essence is drawing political districts in such a way to favor a particular party. This can be accomplished by limiting the voting power of a district and also by spreading the opposite party supporters across numerous districts. This can be used to keep one party in power even though they have the minority.