ANSWER
[tex]3( {x}^{2} - 7)[/tex]
EXPLANATION
The given expression is
[tex]3 {x}^{2} - 21[/tex]
The highest common factor is 3.
Factor 3 to obtain,
[tex]3( {x}^{2} - 7)[/tex]
We could have imposed difference of two squares on the expression in the parenthesis to factor further. But the result is not part of the given options.
Hence the correct answer is A.
