The correct answer is (a) 17.
To convert a binary number to decimal, you multiply each digit by the power of 2 corresponding to its position, starting from the right. The position of the rightmost digit is 2^0, the position of the next digit to the left is 2^1, and so on.
In the binary number 10101, the rightmost digit is 1, so we multiply it by 2^0. The next digit to the left is 0, so we multiply it by 2^1. The next digit to the left is 1, so we multiply it by 2^2. The next digit to the left is 0, so we multiply it by 2^3. And the last digit to the left is 1, so we multiply it by 2^4.
1 * 2^0 = 1
0 * 2^1 = 0
1 * 2^2 = 4
0 * 2^3 = 0
1 * 2^4 = 16
Adding up the products of each digit and its corresponding power of 2, we get 1 + 0 + 4 + 0 + 16 = 17.
Therefore, the decimal equivalent of the binary number 10101 is 17.
Option (b), 37, is incorrect because 37 is not the decimal equivalent of the binary number 10101.
Option (c), 21, is incorrect because 21 is not the decimal equivalent of the binary number 10101.
Option (d), 23, is incorrect because 23 is not the decimal equivalent of the binary number 10101.