Answer:
a. CS begins before the US and ends before the onset of the US.
Step-by-step explanation:
Trace conditioning: In psychology, the term "trace conditioning" is described as one of the types of "classical conditioning" whereby the UCS or unconditioned stimulus & CS or conditioned stimulus are being presented individually with a specific time-interval in-between.
In other words, in "trace conditioning", the conditioned stimulus and unconditioned stimulus are being isolated in time by a "trace interval".
In the question above, the given statement represents option-a as the correct answer.