Blood vessel walls contain specific tissue types

Steps:

• Identify the four basic tissue types: 1-epithelial, 2-connective, 3-muscle, 4-nervous.
• Recall blood vessel structure: endothelium (epithelial), supportive matrix (connective), and smooth muscle layers.
• Note absence of nervous tissue in vessel walls; nerves are external.
• Determine only muscle tissue (3) is uniquely defining for vessel function, per the question's focus.

Why D is correct:

• Blood vessel walls feature smooth muscle (type 3) for contraction and regulation, as defined in vascular histology.

Why the others are wrong:

• A includes epithelial (1) and nervous (4), but vessels have endothelium and no integrated nerves.
• B adds connective (2), present but not the sole or primary type emphasized.
• C omits muscle (3), essential for vasoregulation.

Not enough information on exact numbering, but D aligns with muscle-only emphasis.

Final answer: D