Van't Hoff factor (i) is given by the expression:

a. $i=\frac{\text{ Normal molar mass }}{\text{abnormal molar mass.}}$

b. $i=\frac{\text{ Abnormal molar mass }}{\text{normal molar mass. }}$

c. $i=\frac{\text{ Observed colligative property }}{\text{calculated colligative property. }}$

d. $i=\frac{\text{ Calculated colligative property }}{\text{observed colligative property.}}$

The correct choice among the given options are:

1. (a, b)

2. (b, c)

3. (c, d)

4. (a, c)