[Hero]

The correct answer is D. Squamous metaplasia is a frequent airway response to chronic irritation from cigarette smoking. It represents the replacement of one differentiated tissue with another mature, differentiated tissue. It is believed that the squamous epithelium is more resistant to injurious agents, and so represents an adaptive response.
Dysplasia (choice A) is characterized by pleomorphism (the individual cells may vary widely in appearance) and by loss of normal tissue architecture, rather than by replacement with another mature epithelium (squamous).

Hyperplasia (choice B) refers to growth of a tissue or organ by cellular proliferation.

Malignant transformation (choice C) is the multistep process by which normal cells progress to the "cancer" phenotype. Metaplasia is not malignant transformation, although the stimuli producing metaplasia may also be associated with the development of malignancy.

Necrosis and repair (choice E) typically result in re-epithelialization and/or fibrosis, not metaplasia.