**Core Concept**
Phase II clinical trials are a critical phase in the development of new drugs. They are designed to assess the efficacy and safety of a drug in a larger population than Phase I, typically involving 100 to 300 participants. This phase aims to determine the optimal dosage and identify potential side effects.

**Why the Correct Answer is Right**
The primary objective of Phase II is to evaluate the efficacy of the drug in a larger population. This involves assessing the drug's ability to produce a desired therapeutic effect, such as reducing symptoms or slowing disease progression. In addition to efficacy, Phase II trials also aim to gather more information on the drug's safety profile, including identifying potential side effects and adverse reactions. This is typically achieved through randomized controlled trials, where participants are randomly assigned to receive either the experimental drug or a placebo.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because Phase I trials are primarily focused on assessing the safety and tolerability of a drug in a small population, not its efficacy.
**Option B:** This option is incorrect because Phase III trials are designed to confirm the efficacy of a drug in a larger population and compare it to existing treatments.
**Option C:** This option is incorrect because Phase I trials are primarily focused on assessing the safety and tolerability of a drug in a small population, not its efficacy.

**Clinical Pearl / High-Yield Fact**
It's essential to remember that Phase II trials are a critical phase in the development of new drugs, as they provide valuable information on both efficacy and safety. A well-designed Phase II trial can make or break a drug's chances of success in subsequent phases.

**Correct Answer: C. Efficacy and safety in a larger population.**