To solve these types of questions you are looking for a similarity of form or structure between 2 clue symbols and looking to replicate these elements in an answer symbol.
The word 'Isomorphic' is from Ancient Greek, with 'isos' meaning equal and 'morphe' meaning shape>
You need to look at the elements of the symbols in relation to outlines, shading, line thickness, orientation (i.e. which direction the symbol is ‘pointing’), size, and position in relation to other parts of the main symbol. The number of each of these elements is important too.
In fact, you need to observe all the usual NVR suspects!
What observations do we notice about these usual suspects in this example?
What logical steps can we make when we apply these observations to the code letters?