What is sandwiched between the layer of circular and longitudinal muscle in the muscularis externa?

The Auerbach plexus or myenteric plexus.

Explanation: The muscularis externa is a composed layer of muscles, located throughout the gastrointestinal tract. It has an inner circular muscular layer and a longitudinal external muscular layer. Between the two layers there is a part of the nervous system called Myenteric plexus.  

A plexus is like a web of axons (nervous tissue) that provides a specific function. In this case, the myenteric plexus has a main function: motility. All the digestive tract (esophagus, stomach and intestine) possess this structure, to give motility or “peristalsis”, so the food could travel all the way from the esophagus through the colon.  

answer; i do believe that the correct answer is; ///mendelian genes are at specific loci on the chromosome and, in turn, segregate during meiosis;

b. lithosphere, asthenosphere, mesosphere   is the answer most likely