The purpose of a double-blind study is to: avoid observer and subject bias avoid observer bias and sampling variation avoid subject bias and sampling variation achieve comparability of treated and untreated subjects

Avoid observer and subject bias

Explanation:

Double-blind study is a human clinical trial method where neither the examiner nor the examiner knows what is being used as a variable at any given time. The purpose of the double-blind test is to avoid any conscious or unintentional interference with the results of an experiment and thus to avoid observer or subject bias in the result of the experiment.

i think it is the third one.