The correct answer is: A. Radial shakes.
Radial shakes are cracks that occur in the wood perpendicular to the growth rings. They are caused by shrinkage of the exterior surface of the wood exposed to the atmosphere. The wood shrinks more on the outside than on the inside, which causes the wood to split.
Heart shakes are cracks that occur in the heartwood of the tree. They are caused by a number of factors, including drying stresses, decay, and insect damage.
Wind cracks are cracks that occur in the wood when it is subjected to strong winds. The wind causes the wood to bend and twist, which can cause the wood to crack.
Twisted fibres are a type of defect that occurs in the wood when the fibres are not aligned properly. This can be caused by a number of factors, including improper drying, handling, and machining.