1. Write the dissociation of the salt

For the salt $MX_2$ in aqueous solution:

$MX_2 \rightarrow M^{2+} + 2X^-$

So, on complete dissociation, 1 formula unit gives 3 particles.

2. Use the van’t Hoff factor formula

If the degree of dissociation is $\alpha$, then for a solute that gives $\nu$ particles on dissociation,

$i = 1 + (\nu - 1)\alpha$

Here, $\nu = 3$, so

$i = 1 + (3-1)\alpha = 1 + 2\alpha$

3. Substitute the given value of $i$

Given van’t Hoff factor:

$i = 2$

Thus,

$2 = 1 + 2\alpha$

$2\alpha = 1$

$\alpha = 0.50$

4. Match with the options

$\alpha = 0.50$

So the correct option is B.