Concept: Climate change mitigation means reducing greenhouse gas emissions or increasing carbon sinks. It includes renewable energy, energy efficiency, carbon trading, afforestation, and cleaner technologies.
Step 1: Check mitigation measures.
Use of alternative energy is linked to mitigation because solar, wind, hydro, and other clean energy sources reduce fossil fuel use.
\[
A = \text{Mitigation measure}
\]
Carbon trading is linked to mitigation because it creates an economic mechanism to reduce carbon emissions.
\[
C = \text{Mitigation measure}
\]
Reduction of energy consumption is also linked to mitigation because less energy use generally means fewer emissions.
\[
D = \text{Mitigation measure}
\]
Step 2: Identify the option not directly linked.
Reduction in population is not generally treated as a direct climate change mitigation policy measure in the same way as renewable energy, carbon trading, and energy conservation.
\[
B = \text{Not a standard mitigation measure}
\]
Therefore, the correct answer is B Only.