Step 1: Understand the format.
This is an Assertion-Reason item; we judge each part and their link.
Step 2: Evaluate the Assertion.
A teacher's positive attitude improves the classroom learning environment through support and encouragement. This is true.
Step 3: Evaluate the Reason.
Attitude is a psychological predisposition that shapes a person's behaviour and responses. This is also true.
Step 4: Test the causal link.
Does attitude influencing behaviour explain why a positive attitude improves the classroom- Yes, the supportive behaviour driven by attitude is precisely what creates the better climate.
Step 5: Reject the partial options.
Options C and D fail because both statements are true, and option B fails because the Reason does explain the Assertion.
Step 6: Choose the full match.
Both are true and the Reason correctly explains the Assertion.
Step 7: Conclude.
This is option A.
\[ \boxed{\text{A. Both [As] and [R] are true, and [R] is the correct explanation of [As].}} \]