In this micrograph depicting the bladder wall, where would stored urine be found?

I would say within the space C

(A) cells are the transitional epithelium cells, there resist to the acidity of the urine. It protects smooth muscle cells (B) responsible for the bladder contraction and the release of the urine by the urethra.
the C space represents the intern cavity of the bladder where the urine is supposed to be stored waiting for the micturition (urination).