(1) x^2+2 x (2) x^2-4 (3) x^2+8 x+15 (4) 6 x^2-2 a x-3 x y+a y (5) 2 x^2+7 x+3 (6) 4 x^2+9 x+5

The correct answer is (4) \( 6 x^{2}-2 a x-3 x y+a y \).<br /><br />This expression can be factored by grouping. First, we can group the terms as follows: \( (6x^2 - 3xy) + (-2ax + ay) \). Then, we can factor out the common factors from each group: \( 3x(2x - y) - a(2x - y) \). Finally, we can factor out the common binomial factor \( (2x - y) \) to get \( (3x - a)(2x - y) \).