Which political reform gave people the power to approve or reject laws passed by a legislature?

The correct answer to this question would be Referendum.

A Referendum is a direct vote, where the entire population is invited to vote on a particular proposal. This may cause the adoption or rejection of a new law, proposal, amendment, depending on what the referendum refers to.

It's a very important tool for Democracy, as it allows people not only to speak their minds, but be active in the decision making of a nation.

Referendum is the political reform that gave people the power to approve or reject laws.