The appearance
P : this dinosaurs were at their peak
Q : of the first mammals on the earth
R : at the time when
S : went almost unnoticed

The Proper sequence should be:

A. SRPQ

B. QSRP

C. QRPS

D. RPQS

Answer: Option C

Solution(By Examveda Team)

The appearance of the first mammals on the earth at the time when this dinosaurs were at their peak went almost unnoticed.