In every row the third grid is made from the first two grids. Find how, and then select the grid that completes the bottom row.

Lay the first two grids on top of each other: a cell shaded in just one of them stays shaded, and a cell shaded in both becomes white. In the bottom row the top-middle cell and the bottom-left cell are shaded in both, so they drop out, leaving the top-left and centre cells: (a). (b) keeps every shaded cell, (c) keeps only the shared cells and (d) is not formed from the two grids.