Right choice is (a) (11.6875)10
The best I can explain: Binary to Decimal conversion is obtained by multiplying 2 to the power of base index along with the value at that index position.
(01011)2 = 0 * 2^4 + 1 * 2^3 + 0 * 2^2 + 1 * 2^1 + 1 * 2^0 = 11
(1011)2 = 1 * 2^-1 + 0 * 2^-2 + 1 * 2^-3 + 1 * 2^-4 = 0.6875
So, (01011.1011)2 = (11.6875)10.