Astronauts appear weightless while working in the international space station because a) they are in a state of free fall. b) there isn't any gravity in outer space. c) they have very little mass when in outer space. d) they have gravitational attractions to the sun and moon.

A is correct.  There is plenty of gravity in outer space.  That is what keeps the space station in orbit, as well as the moon, which is a lot farther away than the space station.  The state of free fall is what orbiting actually means.  The space station is falling down toward the earth, but due to its (fast) motion horizontal to the earth's surface, it's motion isn't straight down.  In fact, as it moves horizontally, it falls down at the same rate that the curved surface of the earth "falls away" underneath it.  So unlike most projectiles that fall with a curved trajectory and hit the ground, things in orbit have a curvature to their trajectory that matches the curvature of the earth.  So instead of hitting the ground, the ground is curving away at the same rate the object falls.
Since all things fall at the same rate, the space station, astronauts, and everything else inside are all falling at the same rate and so they feel weightless.  This would be the same in an elevator that was in free fall on earth, but since that would only be moving downward, it would eventually crash into the ground.