Which uterine layer is composed of a simple columnar epithelium containing tubular glands?

The correct answer is endometrium.

The endometrium is one of the three layers of uterine (myometrium and perimetrium are another two). It is the inner epithelial layer which contains a basal layer and a functional layer. The endometrium is the inner epithelial layer, along with its mucous membrane, of the mammalian uterus. It has a basal layer and a functional layer. The functional layer thickens and then sheds during the menstrual cycle. During the pregnancy, the uterine glands (tubular) of the functional layer increase in size and number.

The uterine layer that is composed of a simple columnar epithelium containing tubular glands is the endometrium. It is the mucous membrane that lines the inside of the uterus. It is the innermost glandular layer and functions as a lining for the uterus, preventing adhesions between the opposed walls of the myometrium and hence maintaining the patency of the uterine cavity. 

The answer is constant.