Which one of these four numbers is a palindrome both in ordinary decimal representation and in binary representation?
A palindrome is something that reads the same forwards and backwards.
The number 33 obviously reads the same forwards and backwards.
In binary representation the number 33 is represented as 100001. This also reads the same forwards and backwards.
But 11, although a palindrome in decimal representation is 1011 in binary (not a palindrome); so also 55 (which reads as 110111 in binary) and 77 (which reads as 1001101 in binary).