Frequency = 1 mega hertz so f = 1 MHz
so let the number of bits in the counter is n ,
1024 = 2n so n = 10
f = (1/ n * Td ) [ Td is propagation delay ]
= > Td = 1/nf
= 1/ ( 10 * 1 * 106 )
= 100 * 10-9
= 100 nanoseconds [ as 1 nano sec = 10−9 second ]
hence Td = 100ns .