I always have trouble differentiating power and validity but power is related to precision (P for P) and validity is related to accuracy.

Precision or reliability is the consistency or the reproducibility of a test and increases as the statistical power increases (the probability of NOT making a type II false-negative error). You can increase the power (the probability of rejecting the null hypothesis when it is false) by

• increasing the sample size
• increasing the expected effect size
• increasing the precision of measurement

(C) The power of the study is increased would be a good answer if we increased the size of the study or made more precise measurements, or if we made the study more reproducible.

Accuracy or validity can be increased by decreasing systematic errors in our tests or by reducing biases (biases are just mechanisms that push us away from the true values).

I personally think that their chosen answer (A) The effect of confounding variables is decreased would be a better answer if it mentioned that Selection bias or Sampling bias was reduced/eliminated.