Assertion : Castes were traditionally linked to occupations and hence caste system functions as the social division of labour.
Reason: Mobility is allowed in the caste system.

A is true but R is false

The correct answer is option 4: A is true but R is false

Assertion : Castes were traditionally linked to occupations and hence caste system functions as the social division of labour.  This is correct. Castes were traditionally linked to occupations. A person born into a caste could only practice the occupation associated with that caste, so that occupations were hereditary, i.e. passed on from generation to generation. 
Reason: Mobility is allowed in the caste system. This is false. Castes are not only unequal to each other in ritual terms, they are also supposed to be complementary and non-competing groups. In other words, each caste has its own place in the system which cannot be taken by any other caste. Since caste is also linked with occupation, the system functions as the social division of labour, except that, in principle, it allows no mobility.