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

a. i= Normal molar mass  abnormal molar mass.

b. i= Abnormal molar mass  normal molar mass. 

c. i= Observed colligative property  calculated colligative property. 

d. i= Calculated colligative property  observed colligative property.

The correct choice among the given options are:

1. (a, b)

2. (b, c)

3. (c, d)

4. (a, c)

Hint: Van't Hoff factor (i) i= normal molar mass  abnormal molar mass 
van't Hoff factor (i) is a measure of the extent of association or dissociation of solute particles which can be calculated as follows:
normal molar mass / abnormal molar mass