Step 1: Recall the sexual response cycle: desire, arousal/excitement, orgasm (with ejaculation in the male) and resolution.
Step 2: Place premature ejaculation. Ejaculation is the event of the orgasm phase. Premature ejaculation is a disorder of ejaculation, so it is an orgasm-phase dysfunction.
Step 3: Why the others are wrong. Erectile dysfunction belongs to the excitement/arousal phase, the plateau is sustained arousal before orgasm, and the refractory period follows orgasm. Since the abnormality is the timing of ejaculation itself, it falls in the orgasm phase.