Step 1: What is being asked.
We must match the Marxian theory of social inequality with the right school of thought in geography.
Step 2: Idea we will use.
Each school of thought in geography is known by its main idea. Marxian ideas about class and inequality are linked with only one school. We match each choice with its known focus.
Step 3: Test choice 1.
Choice 1, Positivist, relies on numbers, laws and spatial models. Its focus is measurement, not class conflict, so it is not the answer.
Step 4: Test choice 2.
Choice 2, Humanistic, places humans at the centre and studies their feelings, values and meaning of place. This is not a Marxian approach.
Step 5: Test choice 3.
Choice 3, Radical, came up when geographers wanted to explain poverty and inequality. They took help from Marx's ideas on class and capital. This matches the question.
Step 6: Test choice 4.
Choice 4, Behavioural, looks at mental maps and the way people see their surroundings. Class conflict is not its theme, so it does not match.
Final Answer:
The Radical School of Thought used Marxian theory, so option 3 is correct.
\[ \boxed{\text{Option 3}} \]