A binomial with degree 2 is a binomial squared, square binomial (or binomials squared) are those in which the sum or subtraction of two terms must be raised to the power of two.
Among the 4 options, the one that meets these criteria would be the following one.
[tex]x^2-9=x^2-3^2[/tex]Evidently here we have a subtraction between two terms raised to the power of 2.
In conclusion, the answer is:
[tex]x^2-9[/tex]