Amongst given options, macular edema, vitreous hemorrhage and retinal traction (tractional retinal detachment) are common causes of vision loss in diabetic retinopathy. Vitreous hemorrhage and tractional retinal detachment occur in Proliferative diabetic retinopathy (while the patient in question has non-proliferative diabetic retinopathy). So, we are left with macular edema, which is the most common cause of vision loss in NPDR. However you should keep in mind a very important fact that macular edema usually causes gradual loss of vision and sometimes it may cause sudden loss of vision."Macular edema, a common cause of gradual loss of vision in diabetics" . — Practical ophthalmics by peter simmock "Macular edema may cause sudden painless loss of vision, However, a patient with macular edema is likely to have blurred vision with gradual loss of vision".  — Basics of ophthalmics