3. In the United States, third-party movements occur
most often when
A. there is clear danger of foreign attacks
B. the President is inconsistent in foreign policy
their
C. major political parties ignore vital public
issues
D. interest in overseas trade and possessions
intensifies

C. major political parties ignore vital issues

Explanation:

Currently, there's only 2 major political parties in United States. Democrats and Republicans. Any party other than those two often considered as "third-party".

One Example a big third party movement is the Green party.

This party gained momentum in 2001 due to the raising concern of Climate Change in Untied States. But, none of the major political party make active legislations to address the problem.