The correct answer is:
(d) Dust
In foundries, the presence of silica dust is a major health hazard and can lead to serious lung diseases, such as silicosis. Workers who are exposed to respirable crystalline silica dust can develop respiratory problems over time, making dust exposure the primary hazard responsible for lung diseases in foundries.
Noise, metal splashes, and heat are also hazards but are not directly associated with lung disease in the same way that dust is.