Environmental Science Question: What is a "risk-assessment model" and what does it do?

For the multiple choices (I am not a fan) B is probably what the grader expects.

A risk assessment looks at a defined situation (location, boundary, time period...)

It begins by identifying Hazards (e.g., sources of contamination, such as lead in soils)

It then identifies pathways by which humans or ecological receptors could be exposed. e.g., humans drinking tap water that has been contaminated by lead from old plumbing.)

It then assesses the impact of the exposure, usually by looking at the dose that is possible (e.g., mg L per kg of human mass in the receiver). It then uses published/peer reviewed data to look at the consequence of that dose in terms of known responses (e.g., 50% of receivers who drink more than X liters of water containing Y mg/L Pb die...)

Then, in the more sophisticated versions, the assessment considers the probabilities associated with each of those steps Risk = Consequence * Probability

If the risk is too high to protect the public, then , it considers how the system might be mitigated (e.g., by replacing old pipe that used Pb solder with plastic pipe).

If you need something simpler than that, you are just out of luck.

Finally, it does the arithmetic implied by these steps and determines a level of Pb that would be acceptable in terms of risk to human health and the environment.

There are lots of areas in which risk assessment can/should be used. It is the basis of insurance rates, for instance. It could be applied, rather easily to population questions, and is use din many policy decisions (should a new power plant be coal-fired, gas-fired, or nuclear?)