Ans. is 'a' Metaplasia Metaplasia is an abnormality of cellular differentiation in which one type of mature cell is replaced by different type of mature cell and the latter is not normal for the tissue involved.Normally no squamous epithelial cells are found in lungs & respiratory tracts, so any stratified squamous epithelial cells in bronchus occur due to metaplasia.Squamous Metaplasia - is the most common type of epithelial metaplasia- non squamous pseudostratified columnar or cuboidal epithelium is replaced by a normal appearing stratified squamous epithelium.It is common in Endocervix and bronchial mucosa.Glandular metaplasia - It occurs in esophagus*, squamous epithelium replaced by glandular epithelium.