Answer:
11111111=FF.
Explanation:
The following binary number can be converted to hexadecimal number by forming a group of 4 bits and finding the corresponding Hexa decimal number for that group.Start making the group from LSB(Least Significant Bit) to MSB(Most Significant Bit).If the bits are less than 4 in the last group add corresponding number of 0's to the front.
1 1 1 1 1 1 1 1
F F
Hence the answer is FF.