Which of the following is true about the morula?

The morula is indeed characterized by being a solid ball of cells, formed through the process of cleavage following fertilization. At this stage, the zygote has undergone multiple rounds of cell division, leading to the creation of a cluster of cells that resemble a mulberry, which is where the name "morula" originates.

This stage occurs after several divisions of the fertilized egg but before it develops into a blastocyst, which is the stage that implants into the uterine wall. The morula, therefore, is not a single cell but a structure made up of numerous cells, typically about 16 to 32 cells depending on the timing of the divisions.

The other options are less accurate in describing the morula: it is not the first visible stage of development, as the zygote stage comes first; it is not formed after implantation, as implantation occurs at a later stage; and it certainly does not refer to a single cell, as by the time the morula is formed, cell division has already produced multiple cells.