Final answer:
Weak airflow in forced-air systems is typically caused by issues like clogged evaporator coils, dirty filters, or slipping fan belts, but not by undersized burners which affect heating, not airflow.
Step-by-step explanation:
The cause of weak airflow on forced-air systems at the registers is not related to A. Undersized burners. This is because burners are related to the heating process itself, not airflow. However, the other options mentioned, such as B. Clogged evaporator coil, C. Dirty filter, and D. Slipping fan belt on the fan motor, can indeed affect the airflow within a forced-air system. For instance, a clogged evaporator coil can restrict air from passing through it effectively, while a dirty filter can block the air. Additionally, a slipping fan belt will impair the fan motor's function, reducing its ability to circulate air properly. To maintain optimal performance, it is advisable to change filters regularly as central heaters and air conditioners rely on these filters to trap dust and other pollutants.