Convert the decimal (base-10) number 20 to binary (base-2).

To convert the decimal number 20 to binary, we can divide the number by 2 and record the remainders. Here’s how the conversion works:

1. 20 divided by 2 is 10, remainder 0
2. 10 divided by 2 is 5, remainder 0
3. 5 divided by 2 is 2, remainder 1
4. 2 divided by 2 is 1, remainder 0
5. 1 divided by 2 is 0, remainder 1

Now we read the remainders from bottom to top: 10100.

Therefore, the binary representation of the decimal number 20 is **10100**.

The correct answer is:
**D. 10100**