The organizers of the March on Washington for Jobs and Freedom wereA) trying to build support for the civil rights movement // B) members of the...

The best answer of the options that you have given us is
A.  The major purpose of the March on Washington was to get more support for the Civil
Rights Movement.

The organizers of this march were not
unemployed African Americans.  Instead, the march was organized by people from many
different organizations.  The march was first proposed, for example, by a labor union
leader, not an unemployed person or advocate for the
unemployed.

The Nation of Islam did not really support the
March on Washington.  Malcolm X criticized it, for example, because of the fact that it
included white people.  This went against his black nationalist
ideals.

For these reasons, the best answer is
A.