High viscosity fuel oil is challenging to work with because:

High viscosity fuel oil is indeed challenging to work with primarily because it is hard to pump. Viscosity refers to the thickness or resistance to flow of a liquid. In the case of fuel oils, higher viscosity indicates that the oil is thicker and more resistant to flow under gravity. This presents significant difficulties in pumping it from storage tanks to burners or other equipment, as standard pumping equipment may not effectively move the viscous fluid. This can lead to operational inefficiencies and may necessitate the use of specialized heating systems or pumps to reduce the viscosity and allow for easier handling and combustion.

Understanding the characteristics of high viscosity fuel oils is crucial for maintaining efficient operations in heating systems, as it informs decisions around storage, transfer, and combustion processes. In contrast, the other options, while related to fuel properties and combustion characteristics, do not address the fundamental mechanical challenges of handling high viscosity fuels as directly as the difficulty in pumping does.