The correct answer is 3, which means that assertion A is true and reason R is false.

Let`s look at each part:

Assertion (A) says that unsaturated fats are more reactive compared with the saturated fats. This is true because unsaturated fats contains double bonds in their structure which can be broken more easily than the single bonds in saturated fats.

Reason (R) states that unsaturated fats have only single bonds in their structure. This is incorrect. As mentioned before, unsaturated fats contain one or more double or triple bonds in their structure. It`s the saturated fats that have single bonds only.

Therefore, assertion A is correct, indicating that unsaturated fats are indeed more reactive, but reason R is incorrect because it wrongly describes the structure of unsaturated fats. So, the correct answer is option 3: A is true but R is false.