An early shell-type boiler had ___________ as its heating surface?

The correct answer highlights that an early shell-type boiler utilized only the lower part of the shell as its heating surface. This design approach was common in early boilers, where heat transfer was primarily concentrated in the lower section due to the placement of the burner and hot gases rising to heat that specific area.

In contrast, other options describe configurations that involve additional heating surfaces beyond just the lower portion of the shell. For instance, the inclusion of the rear plate or head in other options suggests an increased amount of heat exchange surface area, which was not characteristic of early designs. Similarly, configurations that encompass the total shell side or multiple passes would correspond to more advanced boiler designs that were developed after the initial shell-type constructions. Understanding this developmental context helps to clarify why the focus was on the lower part of the shell as the primary heating surface in early designs.