To make a salad using only cruciferous vegetables, Ariana should not include spinach.
To determine what Ariana should NOT include in the salad, we need to identify the cruciferous vegetables. Cruciferous vegetables include cabbage, cauliflower, and Brussels sprouts. Spinach is not a cruciferous vegetable, so Ariana should NOT include spinach in her salad. Therefore, the correct answer is "O spinach."