Where time = max(stage delays) + buffer delay.
P1 is 4 stage pipeline and only 3 stage delays are given . ( data insufficient ) But with given data time is 2+0.5 = 2.05 ns
P2 time = 1ns + 1ns = 2ns
P3 time = 2ns + 0ns = 2ns
P4 time = 1ns + 0.5ns = 1.5 ns
Frequency = 1 /time
P2 and P3 having same frequency .
Highest Frequency is for less time delay.
P4 is the answer