Assertion (A): Yamuna becomes a dead river between Delhi and Agra most of the year. Reason (R): Yamuna is a non-perennia
In Assertion-Reason, check both parts calmly. Here, the combination that fits is (A) is true, but (R) is false. In Ganges Drainage System, 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) 2015] | Majid Husain, Geography of India, and NCERT Geography textbooks discuss Ganges Drainage System and explain the Indian geographical features and patterns that support the correct answer for this question.