Which of the following is not something a state government can regulate?

marriage

treaties

the death penalty

abortions

A state government can't regulate treaties, the Article II, Section 2 of the Constitution gives that power to the President, as long as he (she) has the senate's concurrence. The Supremacy Clause provides that treaties count as "the supreme law of the land", and as such, states can't interfere in this federal matter.

The state government cannot regulate treaties. treaties are somthing on a federal level.