According to the Partnership Act, each partner acts as a principal when representing their own interests and as an agent when representing the interests of other partners.
The definition in the Partnership Act clarifies that the business can be conducted by all partners collectively or by any one partner acting on behalf of all.
This directly explains Assertion (A), as every partner has a dual role in the partnership business.
Hence, both the Assertion (A) and the Reason (R) are correct, and Reason (R) appropriately explains Assertion (A).