Final answer:
The fourth factor affecting the survival and reproduction of food microorganisms, along with temperature, humidity, and acidity, is salt concentration. Salinity levels influence microorganisms' growth in food, with higher salt concentrations typically inhibiting their development.
Step-by-step explanation:
The correct answer to the question about factors affecting the survival and reproduction of food microorganisms is c) Salt concentration. Besides temperature, humidity, and acidity, microorganisms also require specific salinity levels for growth.
Available water, or water activity (aw), is critical as it influences the environment's osmotic pressure. Bacteria typically require high aw to thrive, whereas fungi may tolerate drier conditions. The control of moisture through drying or adding hypertonic substances like salts and sugars is a common method to inhibit the growth of microorganisms in food, making salinity a key environmental condition that affects microbial growth.