**Core Concept**
Cohort studies are a type of observational study design where a group of individuals (cohort) is followed over time to examine the development of outcomes or diseases. This study design allows researchers to investigate the temporal relationship between exposure and outcome, making it useful for determining causality.

**Why the Correct Answer is Right**
Cohort studies are not particularly useful for rare diseases because they require a large sample size and a long follow-up period to detect significant associations. Rare diseases are often difficult to study using cohort designs due to the limited number of cases, which can lead to inadequate statistical power. Additionally, rare diseases may not be as prevalent in the population, making it challenging to identify sufficient cases for analysis.

**Why Each Wrong Option is Incorrect**
**Option A:** Cohort studies are indeed prospective, meaning they involve following a group of individuals forward in time to observe the development of outcomes or diseases.
**Option C:** Cohort studies are necessary for determining incidence, as they allow researchers to track the development of new cases over time. This is in contrast to case-control studies, which examine the association between exposure and disease in individuals who already have the disease.
**Option D:** Cohort studies can be costly, especially if they involve long follow-up periods and large sample sizes. However, this is not a unique characteristic of cohort studies compared to case-control studies.

**Clinical Pearl / High-Yield Fact**
When designing a study to investigate the relationship between exposure and disease, researchers should consider the study's purpose, sample size, and follow-up period to determine the most suitable study design. In general, cohort studies are preferred for investigating rare diseases or outcomes that require a long follow-up period.

**✓ Correct Answer: B. Useful for rare diseases**