Which one of the duty based ethical principles states, "what would happen if everyone acted this way?"

emphasises the value of every human being

Duty-based ethical systems tend to focus on giving equal respect to all human beings.

This provides a basis for human rights - it forces due regard to be given to the interests of a single person even when those are at odds with the interests of a larger group.

says some acts are always wrong

Kantian duty-based ethics says that some things should never be done, no matter what good consequences they produce. This seems to reflect the way some human beings think.

Rossian duty-based ethics modified this to allow various duties to be balanced, which, it could be argued, is an even better fit to the way we think.

provides 'certainty'

Consequentialist ethical theories bring a degree of uncertainty to ethical decision-making, in that no-one can be certain about what consequences will result from a particular action, because the future is unpredictable.

Duty-based ethics don't suffer from this problem because they are concerned with the action itself - if an action is a right action, then a person should do it, if it's a wrong action they shouldn't do it - and providing there is a clear set of moral rules to follow then a person faced with a moral choice should be able to take decisions with reasonable certainty.

Of course things aren't that clear cut. Sometimes consequentialist theories can provide a fair degree of certainty, if the consequences are easily predictable.

Furthermore, rule-based consequentialism provides people with a set of rules that enable them to take moral decisions based on the sort of act they are contemplating.