A survey researcher randomly selected individuals in Canada, Mexico, and the United States and mailed them a survey about North American trade policies. The researcher compared individuals' survey responses in these countries. This researcher used a

This researcher used a "cross-sectional survey design"

Explanation:

A cross-sectional survey gathers data at one point in time to make conclusions about a population of interest (i.e. universe). Cross-sectional surveys have also been represented as snapshots of the demographics they gather data. Panel surveys are typically performed to assess changes in the studied population.

In this a process of sampling where scores are collected at a single point in time. For instance, cross-sectional sampling might include collecting data from individuals of different ages or levels of development to examine behavioral or other variations between them.