The order of rotational symmetry in a rhombus is: 

Given:

Rotational symmetry in a rhombus.

The rotational symmetry of a shape means that when an object is rotated on its axis, the shape of the object looks the same.

Let us see it in a rhombus.

Suppose we have a rhombus ABCD, when we rotate it by $180°$ it looks like this:

The shape has not changed. Now, if we again rotate $180°$ it looks like this:

Now it is the same as the original one. So, we can say that the order of rotational symmetry in a rhombus is$2$.

Hence, option B is correct.