An assumption is something the speaker takes for granted without saying it. In statement and assumption questions you get a short statement, often a decision, notice, advice or advertisement, followed by two or three possible assumptions. You must decide which are implicit, meaning the statement depends on them.
Three ground rules apply. Accept the statement as true, even if you disagree with it. Do not bring in outside knowledge. And remember that an assumption is never written in the statement itself; if an option only repeats the statement, it is not an assumption.
With two assumptions, each is either implicit or not, which gives 2 × 2 = 4 basic outcomes: only I, only II, both, or neither.
Many papers add a fifth option, 'either I or II is implicit', for a total of 5 answer choices. That option is rare in assumption questions and should be chosen only when the statement clearly depends on one of two opposite assumptions without saying which.