Correct Answer  Option 2 : (10101)
2
Concepts:
Convert decimal to binary
Conversion steps:
 Divide the number by 2.
 Get the integer quotient for the next iteration.
 Get the remainder for the binary digit.
 Repeat the steps until the quotient is equal to 0.
Calculation:
Here, we have to find the binary equivalent of 21
2

21


2

10

1

2

5

0

2

2

1


1

0

So, the binary equivalent of 21 is 10101
i.e (21)_{10} = (10101)_{2}