Complete the starship captain's slightly embarrassing log entry by selecting the correct conjunction.

We had to land on the swamp planet ___ our ship's hyperdrive was making a sound like a dying washing machine.

The correct answer is as.

As can be used as a subordinating conjunction meaning "because" to introduce an adverb clause of reason. "So" introduces a result, "because of" requires a noun phrase instead of a full clause, and "nevertheless" shows contrast.