Your question asks what a scientific law is.
Your answer would be B). What scientists expect will always happen under particular circumstances
A scientific law is something that will always happen, it is mostly like a repeat when doing a experiment with certain factors in place. This is also known as a "natural law" because something will naturally happen when it is enacted with the same factors for multiple "trials". This "scientific law" will allow scientist to predict what would happen during other multiple trials that are conducted. Concise data would be received from these trials.
An example of a scientific law is Newton's law of gravitation.
Therefore, your answer would be B.