## Core Concept
A double-blind study is a research design used in clinical trials and experiments where both the participants and the researchers (or evaluators) are unaware of certain critical aspects of the study, typically the treatment assignments. This method helps minimize bias from both sides, enhancing the reliability of the results. The core concept revolves around reducing bias through blinding.

## Why the Correct Answer is Right
In a double-blind study, not only do the participants not know whether they are receiving the active treatment or a placebo (or control treatment), but the researchers and evaluators also do not know who is receiving which. This helps in minimizing any kind of bias that could creep in through the expectations of either the participants or the researchers. For instance, if a researcher knows a participant is receiving an active treatment, they might unintentionally interact differently with that participant or assess their responses differently. By keeping both parties unaware of the group assignments, the study aims to provide more objective results.

## Why Each Wrong Option is Incorrect
- **Option A:** This option is incorrect because it does not specify the correct definition of a double-blind study. Without details, it's hard to assess its accuracy directly, but given that D provides a clear and correct definition related to both participants and investigators being unaware, A can be considered incorrect by the process of elimination and understanding of the term.
- **Option B:** This option might suggest a single-blind study where either the participants or the researchers are unaware, but not both. This does not meet the criteria for a double-blind study.
- **Option C:** Similar to B, if it implies only one side is blinded or another incorrect configuration, it's incorrect.

## Clinical Pearl / High-Yield Fact
A key point to remember is that the purpose of blinding in clinical trials is to reduce bias. A double-blind, placebo-controlled trial is considered the gold standard for evaluating the efficacy of a new treatment. This design helps ensure that the results are due to the treatment itself rather than external factors.

**Correct Answer:** D. Both participant and investigator are unaware of treatment.