The correct answer is C. 21,000.
Factory overhead is a type of indirect cost that is incurred in the production of goods or services. It is not directly attributable to a particular product or service, but is instead incurred in support of the overall production process. Factory overhead can include costs such as depreciation of factory equipment, rent for factory space, and utilities for the factory.
In this question, we are told that factory overhead is Rs. 70,000 and that 70% of it is variable with sales. This means that 30% of factory overhead is fixed, or does not change with sales. Therefore, the amount of fixed part is Rs. 21,000 (70,000 x 30/100).
Option A is incorrect because it is the total amount of factory overhead. Option B is incorrect because it is the total amount of variable factory overhead. Option D is incorrect because it is the total amount of factory overhead plus the total amount of variable factory overhead.