Correct Option
The correct option is Unity.
Explanation
Mole fraction is a concentration term that expresses the ratio of the number of moles of a specific component to the total number of moles of all components present in a solution or mixture. It is a dimensionless quantity denoted by the symbol $x$ (or $\chi$).
Analysis
Consider a solution consisting of multiple components (A, B, C, etc.). If $n_A, n_B, n_C, \dots$ represent the number of moles of each respective component, the total number of moles ($n_{total}$) is the sum of the moles of all individual components:
$$n_{total} = n_A + n_B + n_C + \dots$$
The mole fraction of component A ($x_A$) is defined as:
$$x_A = \frac{n_A}{n_{total}}$$
Similarly, for other components:
$$x_B = \frac{n_B}{n_{total}}, \quad x_C = \frac{n_C}{n_{total}}, \dots$$
When summing the mole fractions of all components in the solution:
$$x_A + x_B + x_C + \dots = \frac{n_A}{n_{total}} + \frac{n_B}{n_{total}} + \frac{n_C}{n_{total}} + \dots$$
$$= \frac{n_A + n_B + n_C + \dots}{n_{total}} = \frac{n_{total}}{n_{total}} = 1$$
Thus, the sum of the mole fractions of all components in a solution is always equal to unity (1).
Key Takeaway: Since mole fraction represents the fractional part of a whole, the sum of the mole fractions of all constituent components in any mixture or solution is always exactly 1.