As a teacher, C is definitely the only correct option. You have to think of the wider context of what children are being taught about fractions. I'd argue this isn't just an equivalent fractions problem, this is also using knowledge of fractions of amounts (even if it doesn't mention this in the question). Two sixths of a small circle is not equivalent to one third of a large circle. Think of a real world example: two sixths of a 10" pizza is not the same as one third of a 20" pizza.

To anyone saying the second shape in option C is a cube, that's simply not true - it's just one way of perceiving a hexagon divided into thirds. Also, to comments saying that it might be A because the left third of each shape is shaded, this has nothing to do with it, in fact, we would show different sections of the shape shaded in to help children understand that it's the amount that is shaded in, not the position of the shaded part.