Answer:
b. Detrusor muscles is the smooth muscle layer.
Step-by-step explanation:
Detrusor muscle is in bundles which are in spiral arrangement. The stretch receptors are placed in these muscles which helps in contraction when the bladder is need to be extended.
This muscles are usually change in length during the passage of excretes where it contracts while voiding and relaxes while filling of bladder. Inner side of this muscles lies lamina propria and urothelium which helps in contractibility and maintaining the tone of the bladder.