$\"\"$

The word $\"\"$ difference $\"\"$ indicates $\"\"$ subtraction operation $\"\"$ and its symbol is $\"\"$ .

The word $\"\"$ is $\"\"$ indicates $\"\"$ equals $\"\"$ and its symbol is $\"\"$ .

The word $\"\"$ the difference of 21 and a number n $\"\"$ represents $\"\"$ .

The word $\"\"$ the difference of 21 and a number n is 6 $\"\"$ represents $\"\"$ . $\"\"$

The option A is correct answer.