For strong electrolytes, van’ t Hoff factor equals the total number of _ ions/atoms in the formula unit.

Hint: Van’t Hoff factor is defined as the ratio of actual number of particles in the solution after dissociation or association to the number of particles for which no ionization takes place. For strong electrolytes van’t Hoff factor is equal to the total number of ions in the formula.

Complete step by step answer:
First of all we will talk about van’t Hoff factor and the colligative properties.
Colligative properties: It is defined as the ratio of number of solute particles to the number of solvent particles. It only depends on the number of ions and not on the nature of the solute.
There are four colligative properties: Depression in freezing point, elevation in boiling point, osmotic pressure and lowering in vapour pressure.
Depression in freezing point: It is defined as the decrease in freezing point of a solvent in addition to non-volatile solute.
Elevation in boiling point: It is defined as an increase in boiling point of a solvent in addition to non-volatile solute.
Osmotic pressure: It is defined as the minimum pressure which needs to be applied to a solution to prevent the inward flow of its pure solvent across a semipermeable membrane.
Relative lowering in vapour pressure: When non- volatile solutes are added to a solution then there is a decrease in its vapour pressure, which is known as relative lowering in vapour pressure.
Van’t Hoff factor: It is defined as the ratio of actual number of particles in the solution after dissociation or association to the number of particles for which no ionization takes place. It is represented by $i$.
In the question we are asked for the Van’t Hoff factor for strong electrolytes. For strong electrolytes (i.e. the compounds which are completely ionised in the solution) van’t Hoff factor is the total number of ions in the compound. For example: let the compound be ${A_x}{B_y}$ and it is strong electrolyte then the van’t factor for this strong electrolyte will be $x + y$ because its number of particles for no ionization is one and number of particles after ionization is $x + y$. So the van’t Hoff factor $i$ is $x + y$.

Note:
Weak electrolytes: The compounds which are not completely ionised in the solution are known as weak electrolytes. They are good conductors of electricity but less than that of strong electrolyte.