**Core Concept**
The comparison of efficacy between new drug A and existing drug B involves understanding the concept of non-inferiority trials, where the new drug is tested against a standard treatment to determine if it is at least as effective as the existing one.

**Why the Correct Answer is Right**
In a non-inferiority trial, the primary objective is to demonstrate that the new drug is not substantially worse than the existing drug. This is typically achieved by setting a margin or a non-inferiority margin, which is the maximum difference in efficacy that the new drug is allowed to have compared to the existing drug. The primary outcome measure is usually a continuous variable, such as blood pressure or blood glucose levels. The trial aims to show that the new drug is within this margin compared to the existing drug.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because a phase II trial primarily focuses on dose-finding and toxicity assessments, rather than comparing efficacy between two treatments. A non-inferiority trial is typically a phase III trial.
**Option B:** This option is incorrect because a randomized controlled trial (RCT) is a study design, not a specific type of trial. While an RCT may be used to compare the efficacy of two treatments, it is not a specific type of trial that is used to compare new and existing treatments.
**Option C:** This option is incorrect because a cross-over trial is a study design where each participant receives both treatments, but it is not a specific type of trial that is used to compare new and existing treatments.

**Clinical Pearl / High-Yield Fact**
When comparing the efficacy of new and existing treatments, it is essential to define a non-inferiority margin that is clinically meaningful and to use a suitable statistical analysis to determine if the new treatment meets this margin.

**Correct Answer:** D.