The correct option is A cristae
The infoldings of mitochondria are known as cristae. Structurally, the mitochondria are bounded by two membranes. i.e. the outer and the inner membrane, separated by a space called outer chamber or intermembrane space. The inner membrane is thrown up into a series of folds called cristae.
Matrix is the fluid enclosed by the mitochondrial cavity. Cisternae are a bunch of flattened membrane-bound sacs. Flattened pigmented membrane-bound sacs in chloroplasts are called as thylakoids. Thylakoids stacked together are called as grana.