On a two component solid- liquid phase diagram, an isopleth indicates which of the following?
A.A region where the composition of the system is constant
B.An area above which only the liquid phase exists
C.A region where the pressure is constant
D.An area below which only the solid phase exists

Hint: To answer this question, you must recall the various parts of the phase diagram drawn for a two component solid- liquid system. A phase diagram is a plot drawn between temperature and the composition of a two component system. An isopleth is a line drawn on the phase diagram.

Complete answer:
Phase diagram is the graphical representation of the variation of physical states of substances plotted under different conditions of temperature and pressure. A typical phase diagram has pressure plotted along the y axis and temperature plotted along the x axis. In case of a two component system, of the two variables pressure and temperature, we put pressure as a constant. Thus, now in the phase diagram, the temperature is plotted on the y axis and the compositions of the components on the x- axis.
An isopleth is a line which indicates the region in which the composition of the system remains constant throughout.

Thus, the correct answer is A.
Note:
The phase diagram is drawn on the basis of the Gibbs phase rule. The phase rule gives a relation between the number of components, phases in the mixture and the degrees of freedom with varying temperature and pressure. The phase rule equation is \[F = C + P - 2\] , where the number two signifies the two variables temperature and pressure. For a two component system, if we draw the phase diagram, it would be plotted as a three dimensional plot. Thus, we assume pressure to be constant and we draw the phase diagram by varying the temperature. The phase rule equation becomes, \[F = C + P - 1\].