In a transverse section of a dicot stem treated with phloroglucinol-HCl, the crimson-stained regions correspond to:

Correct answer: (d) Primary xylem vessels (lignified)

Explanation

Phloroglucinol with hydrochloric acid is a stain for lignin. Lignified cell walls turn crimson or pink-red.

In a section of a dicot stem, the lignified cells are the xylem vessels and tracheids, and the fibres (sclerenchyma), so these regions stain crimson. Phloem, the parenchyma of the pith and the collenchyma of the cortex have no lignin and stay unstained. That makes the stain useful for picking out the xylem.