The correct answer is option D, which states that the secondary growth of woody angiosperm stems occurs by vascular cambium and cork cambium.

Secondary growth is the increase in girth or thickness of a plant stem or root. It occurs in woody plants such as trees and shrubs, allowing them to grow taller and thicker over time. The primary meristems, known as apical meristems, are responsible for primary growth in plants, which results in the increase in length of the stem or root. However, secondary growth is carried out by two lateral meristems, the vascular cambium and cork cambium.

The vascular cambium is a cylindrical layer of meristematic cells located in the stem, just beneath the bark. It is responsible for the production of secondary xylem (wood) towards the inside and secondary phloem towards the outside. The secondary xylem adds to the thickness of the stem and provides mechanical strength, while the secondary phloem transports sugars and other organic compounds throughout the plant.

On the other hand, the cork cambium is a lateral meristem that originates from the cortex or epidermis of the stem. It produces cork cells towards the outside and phelloderm cells towards the inside. The cork cells form the protective layer known as bark, which helps prevent water loss, mechanical damage, and pathogen invasion. The phelloderm cells function as a secondary cortex.

In summary, the secondary growth of woody angiosperm stems occurs as a result of the activity of two lateral meristems, the vascular cambium and cork cambium. The vascular cambium adds secondary xylem and phloem, contributing to the increase in girth, while the cork cambium produces cork cells and phelloderm, forming the protective bark layer. This process allows woody plants to grow thicker and more structurally stable over time.