To Which of the Following Two-Object Systems Can We Apply $C$

Short Answer

To which of the following two-object systems can we apply conservation of momentum, and why? In each case, answer "A" if we can apply conservation of momentum because the system floats in space, "F" if it's because the system is functionally isolated, " $C$ " if it's because the system undergoes a collision, and " $\mathrm{D}$ " if momentum is not conserved at all because the system is not isolated.
-An isolated star passes through a well-defined cloud of gas, and is slowed by friction in this process. (Assume that we can keep track of the cloud's momentum.)