Which research method provides the best way of assessing whether cigarette smoking causes an increase in mental alertness?

The answer is "experiment".

The prime technique for inquiry in science is the experiment. The key highlights are control over variables, cautious estimation, and setting up cause and effect relationships.

An experiment is an examination in which a hypothesis is logically tried. In an examination, an independent variable (the reason) is controlled and the dependent variable (the impact) is estimated; any incidental factors are controlled.

no questions on whether they are allowed that right