Step 1: The assertion states that abscisic acid is a stress hormone, which is true as it is known to regulate responses to stress in plants.
Step 2: The reason provided, that abscisic acid increases the tolerance of plants to various kinds of stresses, is also true. Abscisic acid plays a significant role in plant stress tolerance by inducing various protective mechanisms like stomatal closure, etc.
Step 3: Since both (A) and (R) are true and (R) explains why (A) is true, the correct option is (1).