In the given pressure- temperature diagram, for water, which point indicates triple point?

(A) A
(B) C
(C) P
(D) E

Hint: The triple point of a substance is the temperature and pressure at which its three phases (gas, liquid, and solid) coexist in thermodynamic equilibrium, according to thermodynamics. The temperature and pressure at which the sublimation, fusion, and vaporisation curves all intersect. This point is unique for different substances.

Complete Step By Step Answer:
The triple point is the point at which all three phases of matter coexist. It is easy to convert the matter into another phase at this point by making a small change to the system. In the diagram given above the point at which all the three phases of water, that is, vapour, liquid and solid, meet each other is at the point P. Hence P is the triple point.
Every substance has a unique triple point. The triple point of water is important because it is used as a standard reference point for defining the Kelvin temperature scale. Because the triple point is a point, the three phases can only exist at one temperature and pressure. This information is frequently useful in identifying compounds or solving problems.
The correct answer is option C, P is the triple point of water.

Note:
For substances having many polymorphs, a triple point may involve more than one solid phase in addition to the solid, liquid, and gas phases. Helium-4 is a unique instance since it has a triple point comprising two fluid phases.