In the galactic disk, one would find abundant gas, dust, and active star formation (a), as well as stars of all ages, including O and B stars (b). Therefore, the correct answer is (c).
In the galactic disk, a dynamic and active region within a galaxy, one would expect to find a diverse array of astronomical phenomena. Firstly, the presence of abundant gas and dust characterizes the galactic disk, fostering an environment conducive to ongoing star formation. Regions rich in interstellar material provide the raw materials necessary for the birth of new stars, contributing to the continuous evolution of the galactic population.
Secondly, stars of various ages, including O and B stars, are prevalent in the galactic disk. O and B stars are typically young, massive, and hot, making them prominent members of regions with active star formation. Their presence signifies not only the ongoing birth of stars but also the diverse life cycle of stellar bodies within the galactic plane.
In summary, the galactic disk encapsulates a dynamic interplay of gas, dust, and stars of different ages, creating a rich tapestry of astronomical processes. The coexistence of these elements makes option (c), "All the above," an accurate representation of the expected features within the galactic disk.