Question

question 3
spiral arms
rotate in a fixed pattern, like a pinwheel.
are thought to be maintained by spiral density waves.
get flatter & bluer over time.
get more tightly wound as a galaxy ages.

Explanation:

Spiral arms of galaxies are believed to be maintained by spiral density waves. These density waves compress gas and dust, triggering star - formation and creating the appearance of the spiral arms. They do not rotate like a rigid pinwheel. As a galaxy ages, spiral arms do not generally get flatter and bluer or more tightly wound in the way described in the other options.

Answer:

are thought to be maintained by spiral density waves.