Attorney General Robert Kennedy wanted to end the Freedom Rides, seeking

a.

to avoid alienating white southern voters.

b.

a "cooling-off period."

c.

another racial riot in the Watts section of Los Angeles.

d.

more legislation.

B. A "cooling-off period."

Explanation:

The Freedom Rides, was a movement initiated by a group consisting of twelve African-Americans and six whites, who were commemorating the seventh anniversary of a decision of the Supreme court which ruled that segregation in schools across the country was not constitutional. These groups attempted entering 'whites only' areas and this action, resulted in violent attacks by white mobs. The movement began on May 4, 1961.

When the situation became really critical, Attorney General Robert Kennedy, issued a statement where he advised that a cooling-off period be observed until a state of normalcy is restored. He reasoned that some of the travelers in the buses were innocent people who needed to get to their destinations, and so could easily be attacked by the mob who asks no questions.  

all of the

that would be the steamboat because none of the other things are forms of transportation