Congruence and similarity are very important terms in geometry and indeed the wider field of maths, and are often confused or regarded as interchangeable. Yet there is a key difference.
Congruent means 'identical'. In geometry this means it in the literal sense, i.e. two images are congruent only if they are exactly the same in size and shape. However another example might be modular arithmetic, where two integers can be considered congruent modulo N. This simply means the remainder of these two numbers, when they are each divided by N, is the same. But that's a little tangential in fairness...
Similarity means things are in proportion. Any two squares are similar because by the nature of being a square, they have 4 right angles and equal side lengths. The ratio between any two given side lengths is always the same.
But other shapes require more strictly conditions to be considered similar. For example there is a lot of flexibility in the shape of a triangle, depending on side lengths and angles. Two triangles can only be considered similar if they have the same 'general shape', which is interestingly governed solely by the angle choices. The angles have to be congruent, and then any two triangles with those combinations, whatever size, will be similar and hence have corresponding side lengths in a constant proportion.
The answer is A therefore, as this principle can apply to any polygon. The other answers are pretty inappropriate or nonsensical too.
I hope this helps you :)