Question

A researcher is concerned that gender may affect how subjects respond to an experimental stimulus. so subjects are paired with another subject who has the same gender, and one member from each pair is assigned by a flip of a coin to the experimental group and one to the control group. this method of assigning subjects to groups is called

Answer 1

This method is called: Matching
In this context, matching refers to the pairing of subjects with similar characteristics.
In research, this type of grouping is often used in order to found out whether a certain result is truly influenced by one certain type of variables. (they test all subjects with similar variables in order to recognize if there's a pattern)