The correct answer is:
d. Included liberals and unionists in its definition of "un-american".
Explanation:
The House Un-American Activities Committee was founded in 1938 by the House of Representatives, its main purpose was to detect communist activity within the United States.
The term un-American included liberals and unionists because it referred to anything that was against the nations' political values and culture. During the 1950s fear was spread through the citizens because they were called by the Committee to testify whenever it was suspected they were having un-American behavior, the Committee had the power to investigate American institutions and citizens to prevent communist ideas.