Assertion (A): By 1750 the Maratha kingdom had become a Confederacy presided over by the Peshwa. Reason (R): The success
In Assertion-Reason, check both parts calmly. Here, the combination that fits is Both (A) and (R) are true and (R).... In Maratha State and Confederacy, this is how Assertion and Reason are linked. If you check Assertion first, then Reason, then the link, the answer becomes clear. That is the simple way to do it.
Ref: [UPPCS (Pre) 1997] | Satish Chandra, Medieval India, and NCERT History textbooks discuss Maratha State and Confederacy and explain the political, administrative and cultural developments that support the correct answer for this question.