Ans. is 'a' i.e., Squamous epithelium Metaplasia* Metaplasia is a reversible change in which one differentiated cell type is replaced by another differentiated cell type.Metaplasia represents an adaptive substitution of cells that are sensitive to stress by cell types better able to withstand the adverse environment. Metaplasia may be of two types1. Epithelial metaplasia* Here one type of adult epithelium is replaced by another type of adult epithelium. It may be :-a) Squamous metaplasia# It occurs most frequently in respiratory tract as a result of chronic irritation by cigarette smoking. The columnar epithelium is replaced by squamous epithelium. This squamous metaplasia represents an attempt by the host to repair or prepare an epithelial tissue that has been damaged by environmental toxicants with a more resistant tissue.# Squamous metaplasia is also encountered in other location such as pancreatic duct (Vit A deficiency, stones in pancreatic duct), gall bladder (gall stones), urinary bladder (bladder calculi, Schistosomiasis), and in endocervix.# Though the metaplasia is reversible, if the influences that predispose to metaplasia is persistent, malignant transformation may occur in metaplastic epithelium.b) Glandular metaplasia# It is a change of the squamous epithelium to glandular (columnar) epithelium e.g. development of Barrett's esophagus, i.e., Stratified squamous epithelium of the esophagus is replaced by gastric columnar epithelium. It is due to reflux of acid contents of stomach into esophagus. This necessitates the replacement of susceptible squamous epithelium of the lower third of esophagus with gastric acid resistant glandular epithelium.2. Connective tissue metaplasia* This transformation occurs between the mesodermally derived tissues i.e., one type of connective tissue is replaced by another type of connective tissue. This is characterized by conversion of fibroblast-derived soft tissue into muscle, Cartilage or bone. Example-Bone formation in muscle i.e., myositis ossificans.