The locus of a point in a rhombus which is equidistant from any two adjacent sides coincides with the diagonal of that rhombus.
line segment joining them
Consider a rhombus ABCD and adjacent sides AB and AD.
We know that the locus of a point equidistant from two intersecting lines is the bisector of the angle between them.
So, the locus of a point which is equidistant from AB and AD is the bisector of ∠BAD.
Clearly, it coincides with the diagonal AC.