Concept:
Sports science applies scientific principles related to physiology, biomechanics, psychology, nutrition, and training methods to improve athletic performance.
Performance enhancement may result from several interacting factors such as physical fitness, technical skill, psychological preparation, and behavioural improvements.
Step 1: Examine Assertion (A).
The assertion states that performance improvement mainly occurs due to the application of principles of sports science.
This statement is correct because scientific training and enhancement methods contribute significantly to improved performance.
Step 2: Examine Reason (R).
The reason states that in practical situations it is difficult to identify whether performance improvement is due to fitness or behavioural factors alone.
This statement is also correct because performance depends on multiple variables.
Step 3: Determine whether the reason explains the assertion.
Although both statements are correct, the reason does not directly explain why performance improvement occurs due to sports science principles.
Hence:
\[
\boxed{\text{Both (A) and (R) are correct but (R) is not the correct explanation of (A)}}
\]
Therefore, the correct answer is:
\[
\boxed{(B)}
\]