Assertion (A) : Cattle and goats are never seen browsing on Calotropis growing in abandoned fields. Reason (R) : Cattle and goats avoid grazing on Calotropis because of the offensive smell of its flowers.

The correct option is: Assertion (A) is true, but Reason (R) is false.

Step-by-step Explanation:

1. Analyzing Assertion (A):
The assertion states that cattle and goats are never seen browsing on the plant Calotropis growing in abandoned fields. This statement is biologically true. Grazing animals consistently avoid feeding on this weed in the wild.

2. Analyzing Reason (R):
The reason claims that cattle and goats avoid grazing on Calotropis because of the offensive smell of its flowers. This statement is false. Herbivores do not avoid Calotropis due to the odor of its flowers. Instead, the plant produces a highly toxic chemical substance called cardiac glycosides. If ingested, these glycosides can cause severe cardiac arrest and physiological distress in the animals, which can be fatal. This chemical defense is an evolutionary adaptation of the plant to protect itself from herbivores.

Conclusion:
Since Assertion (A) is true but Reason (R) is false, the correct option is "Assertion (A) is true, but Reason (R) is false."