If an ethernet station collides 4 times then k = 4
So according to exponential backoff algorithm , it has to wait a random amount of time which is given by :
{ 0 to 2k - 1 } * slot time
So here random time = { 0 to 15 } * slot time
= { 0 to 15 } * 51.2
= 0 to 768 [ Only integral multiples of 51.2 is allowed ]
So going by the options :
B) is false as it exceeds 768..
So A) and C) are correct answers