If a polynomial function f(x) has roots -9 and 7-i, what must be a factor of f(x)?

By the conjugate root theorem if we have 7-i as a root we have to have 7+i as a root also.  Written in factor form, they are (x-(7-i)) and (x-(7+i)).  You could rewrite and remove the inner set of parenthesis, but the second of those is what you need.  It's choice one above.