Blood vessels without muscular walls are:

Concept: A capillary may be a blood vessel, which doesn't have the muscular/elastic tissue of other blood vessels. These are the shortest, narrowest and thinnest blood vessels and connect arterioles to venules to complete the circuit. Capillaries consist only of endothelium with some animal tissue binding the cells. Red blood cells squeeze through capillaries file.

Hence, the correct answer is option A.