ANS:A - 10000₂

20 to base 8 means 16 to base 10
i.e. 2 0
2 x 8 raise to 1 + 0 x 8 raise to 0 = 16.

Now 16 base to 10 means 10000 base to 2

i.e. 16/2 = 8 remainder 0
8/2 = 4 remainder 0
4/2 = 2 remainder 0
2/2 = 1 remainder 0
So the answer is 1 0 0 0 0 (the last answer and the remainders, from bottom to top)

Hope it is clear.!!