It would be D:
While I was at the gym—wondering what machine to use next—I ran into my neighbor.
Ludere means to trick. Collusion is secret, illusion is a trick, elude is to avoid escape. These are not options. A prelude is an introduction - almost like a taste to trick you into reading, hearing, seeing more. An ablution is a washing - not connected to a trick at all. Ablution is your answer.
A list of possible answers would help...?
Is a clause that can stand by itself in a simple sentence
