Final answer:
Secondary growth refers to the increase in girth or thickness of stems, particularly in woody plants, and is exemplified by the thickening of a tree trunk due to the activity of the lateral and vascular cambiums.
Step-by-step explanation:
The example of secondary growth given in the options is d) Thickening of tree trunk. Secondary growth refers to the increase in girth or thickness of plant stems or roots, particularly in woody plants. This growth is not related to the elongation of roots (a), increase in stem length (b), formation of lateral roots (c), or increase in leaf number (d). It involves the activity of the lateral meristems, which include the vascular cambium and the cork cambium.
Secondary growth results in the production of secondary xylem and secondary phloem from the vascular cambium, and the replacement of the epidermis with bark from the cork cambium. This causes the stem or trunk of woody plants, such as trees, to become thicker, forming annual rings observable in a cross-section of a tree trunk.