Foods that are capable of supporting rapid growth of micro-organisms are called:

Foods that can promote the growth of microorganisms are classified as Potentially Hazardous foods. These types of food such as eggs, meat, sprouts melons etc are good mediums for the growth pathogens because they contain high levels of protein on which the microorganisms can feed. To prevent the microorganism growth on this food, it should be kept in a temperature controlled environment like the refrigerator which can slow down or inhibit bacterial cell processes. 

option a is correct i.e.

only sodium ions can pass through the membrane.

explanation:

in context to the membrane of nerve impulse, the outer membrane of neuron is semi permeable that is only sodium ions can pass through the membrane i.e. smaller ions can pass through the membrane while larger ions cannot pass through it.

The direction a chemical reaction occurs is represented by an arrow