Which of the following is the correct expression for van't Hoff factor?

Question

$\begin{array}{1 1}(a)\;\large\frac{\text{calculated boiling point}}{\text{observed boiling point}}\\(b)\;\large\frac{\text{calculated osmotic pressure}}{\text{observed osmotic pressure}}\\(c)\;\large\frac{\text{observed colligative property}}{\text{calculated colligative property}}\\(d)\;\large\frac{\text{observed molecular weight}}{\text{calculated molecular weight }}\end{array}$