During the Freedom Rider incident:

a. southern whites adopted the tactics of the civil rights movement and peacefully blocked buses.
b. only the presence of federal troops prevented violence.
c. local police collaborated with the white mobs that attacked buses.
d. white mobs attacked black freedom riders but left white ones alone.

c.local police collaborated with the white mobs that attacked buses.

Explanation:

Freedom Riders were groups of white and African American civil rights  who participated in Freedom Rides in 1961 with the aim to desegregate interstate transportation, including highways, bus stops, and train terminals and to test the Supreme Court's ruling in Boynton v. Virginia (1960), which declared segregation in interstate bus and rail stations unconstitutional.

During the Freedom Rider was met with an incident where local police collaborated with the white mobs that attacked buses in Birmingham, Alabama, where the police chief saw the Freedom Ride as a challenge to his authority in the city.

the first one is d and the second a