**Core Concept**
A cohort study is a type of observational study that follows a group of individuals over time to assess the development of outcomes. The major practical problem in a cohort study is related to the length of time required to observe the outcome of interest.

**Why the Correct Answer is Right**
The major practical problem in a cohort study is **long-term follow-up**. This is because cohort studies require a significant amount of time to observe the development of outcomes, which can lead to issues such as loss to follow-up, attrition, and difficulties in maintaining participant engagement over an extended period. Additionally, cohort studies may also be subject to **confounding variables**, which can affect the validity of the results.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is not a recognized practical problem in cohort studies.
**Option B:** While funding can be a challenge in conducting cohort studies, it is not the major practical problem.
**Option C:** **Bias** can occur in any type of study, including cohort studies, but it is not the primary practical issue.

**Clinical Pearl / High-Yield Fact**
To minimize the effects of long-term follow-up in cohort studies, researchers often use **techniques such as proxy respondents**, **administrative data**, and **electronic follow-up** to maintain participant engagement and reduce loss to follow-up.

**Correct Answer:** D.