Okay, let's tackle this question. The user is asking which branch of sociology studies the relationship between an organism and its environment. First, I need to recall the different branches of sociology. Wait, the question mentions "organism" and "environment," which sounds more like biology or ecology. But the options are sociology branches. Hmm, that might be a trick here.

Sociology typically deals with human society, social relationships, institutions, and culture. The term "organism" usually refers to living beings, so maybe there's a misclassification here. The correct branch in biology would be ecology, which studies organisms and their environments. However, the question is about sociology, so maybe there's a sociology subfield that touches on this.

Wait, maybe the question is mixing up terms. There's a concept called "human ecology" in sociology, which examines how humans interact with their environment, including both physical and social environments. Human ecology might be the right answer here. Let me verify. Yes, human ecology is a branch of sociology that studies the interrelationship between humans and their environment. So the answer should be human ecology. The other options like social stratification, demography, or social psychology don't focus on organism-environment relationships. Ecology is a biology term, so if that's an option, it's incorrect because the question is about sociology. So the correct answer is human ecology within sociology.

**Core Concept**
The question tests knowledge of sociological subfields, specifically those examining human-environment interactions. **Human ecology**, a branch of sociology, analyzes how societies adapt to and modify their physical and social environments. This contrasts with biological ecology, which focuses on non-human organisms.

**Why the Correct Answer is Right**
**Human ecology** studies the dynamic interplay between human populations and their environments, including resource use, urbanization, and social systems. It integrates sociological, economic, and environmental factors to explain patterns like migration, settlement, and sustainability. Unlike biological ecology, it emphasizes cultural and institutional influences on environmental interactions.

**Why Each Wrong Option is Incorrect**
**Option A:** *Social stratification* examines hierarchical divisions in society (e.g., class, caste), unrelated to organism-environment dynamics.
**Option B:** *Demography* focuses on population statistics (age, migration) and trends, not ecological relationships.
**Option C:** *Social psychology* studies individual behavior in social contexts, not environmental interactions.

**Clinical Pearl / High-Yield Fact**
**Human ecology** is often confused with **ecology** (a biological science). Remember: sociology’s "human ecology" = society + environment; biology’s "ecology" = organisms + environment. NEET/USMLE questions often test this distinction.

**Correct Answer: B. Human ecology**

✓ Correct Answer: C. Ecology