The correct answer is: A. Both A and R are true and R is the correct explanation of A.
Ambala receives the highest rainfall in Haryana because it is surrounded by Shivalik hills. The Shivalik hills are a mountain range that runs along the northern edge of the Indian subcontinent. They are part of the Himalayas and are known for their heavy rainfall. The rainfall in Ambala is caused by the orographic effect, which is the lifting of air masses as they rise over mountains. As the air rises, it cools and condenses, forming clouds and rain. The Shivalik hills are high enough to lift the air masses and cause them to cool and condense, resulting in heavy rainfall in Ambala.
The reason (R) is the correct explanation of the assertion (A) because it provides a scientific explanation for why Ambala receives the highest rainfall in Haryana. The reason (R) is also true because the Shivalik hills are known for their heavy rainfall.