Grid A is reflected to give grid B. Reflect grid C in the same mirror line. Which answer grid is correct?

Compare positions: the top-middle cell of A goes to the middle-right in B, and the bottom-left arrow stays in its corner but turns from up to right. That is a reflection in the slanting line from the bottom-left corner to the top-right corner. Reflecting C in that line sends the top-left cell to the bottom-right, the blue bottom-middle cell to the middle-left, and keeps the triangle in the top-right corner but turns it to point down: (d). (a) uses the other diagonal.