Find the missing character?

Answer: Option A

Split the diagram in half, horizontally and vertically, into quarters. Start in the top left cell of each quarter, and move around it in a clockwise spiral towards the centre. Letters are written in alternate segments, moving forward 7 letters at a time for the top left quarter, 6 letters for the top right, 5 letters for the lower right, and 4 letters for the lower left.