I answered this question in the Bromley forum yesterday. I love these Q's and find them easy.
For those who don't know the answer I've copied my explanation below.
Each shape has at least one line (diagonal) stretching from one corner to another — that rules out A
Each shape has a line inside that creates a set of parallel lines — that rules out D
The other line (i.e not the one that creates parallel lines) inside each shape creates a triangle within the shape — that rules out B
This leaves C as the answer