I don't know the answer, but I'm sure someone here can figure it out.
The answer is
Ignore the squares, and just look at the diagonal lines inside the squares.
The first four boxes spell out S,H,E,L.
Box (b) contains an "L" and completes the word to SHELL.
I think the answer is
If you look at the shapes formed by the diagonal lines in each of the diagrams, they form capital letters - S, H, E, L. (b) also contains L while (d) seems to contain C but it makes more sense that it would be spelling the word SHELL rather than SHELC