What are the spiral arms of the milky way made of? a. older stars and comets b. variable stars c. younger stars and gaseous nebulae d. asteroids and comets

The answer is; C.

The spiral arms of the Milky Way galaxy are mainly composed of gas and dust (that make up the nebulae) and young stars. The galaxy has differential rotation across the galaxy with higher rotation speeds closer to the nucleus of the galaxy. The outer spirals rotate slowly and differentially also (akin to traffic jams). When there is a compression due to accumulation of gas and dust, a young star is likely to be formed at the point.