At the triple point, the substance exists in all three phases simultaneously: liquid, solid, and vapor. This unique point occurs at a specific temperature and pressure where the substance can coexist as a solid, liquid, and vapor in equilibrium.

Option 1 states that at the triple point, the substance exists only in the liquid phase. This is incorrect because, at the triple point, all three phases coexist.

Option 2 states that at the triple point, the substance exists in the solid phase only. This is incorrect because, at the triple point, all three phases coexist.

Option 3 states that at the triple point, the substance exists in the vapor phase only. This is incorrect because, at the triple point, all three phases coexist.

Therefore, the correct answer is option 4: the substance coexists in all three phases (liquid, solid, and vapor) at the triple point.