Recall the chapter on research methods in your text. natural improvement, nonspecific treatment effects, and placebo effects are best seen as threats to the of outcome research.

Internal validity.

Explanation:

Internal validity is the study in which trust relationship is build between treatment and outcome. This relationship is known as cause-and-effect.

In this type research, the studies are based on whether the finding is questionable or not. The research which can overrule the possibility of alternative explanations of our findings is internally valid.

The threats of internal valid study are:

So, the correct answer is 'Internal validity'.