Final answer:
Installing a new system fan is the correct solution to mitigate debris and prevent overheating in a computer system by improving airflow.
Step-by-step explanation:
The question relates to mitigating debris and addressing occasional overheating in a computer system by facilitating airflow to cool down the internal components. The correct solution from the provided options would be to install a new system fan. A system fan increases air circulation, reducing the chances of overheating. It does not directly prevent debris accumulation, but it does facilitate airflow which can help keep components clean. In contrast, installing more RAM or a new motherboard or using blanking plates would not serve the same primary purpose of cooling.