Which of the following expressions represents the CIDR subnet mask 255.255.224.0?

A subnet mask of 255.255.224.0 has 8 bits set to on in the first two octets and 3 bits set to on in the third octet. As a result, it is equivalent to /19 in shorthand.
Each bit in the binary number system (base two) can have one of two values:
0 or 1. Each bit in an octet increases by an exponent of two as it moves from right to left, giving the octet bits the decimal values: | 128 |64 |32 |16 | 8 | 4 | 2 | 1 |.
Because subnet mask bits are always utilized from left to right, one bit equals a decimal value of 128. Two bits equals the decimal value 192 (128 + 64).
Three bits would be equivalent to 224 (128 + 64 + 32), and so on.
The comparable decimal value is 255 when all 8 bits of the octet are used for the subnet mask.
In this example, the first two octets (8 bits each) are totally turned on, but the third octet only has three bits turned on, for a total of 19 bits in the subnet mask turned on to indicate the network number (8 + 8 + 3 = 19).