Step 1: Recall the SDGs.
In 2015 the UN adopted 17 Sustainable Development Goals to be reached by 2030.
Step 2: Match the description.
The goal about reducing inequality within and among countries is SDG 10, titled Reduced Inequalities.
Step 3: Check the other numbers.
SDG 6 is Clean Water, SDG 11 is Sustainable Cities, and SDG 12 is Responsible Consumption. None of these focus on inequality.
Step 4: Conclude.
\[ \boxed{\text{SDG 10}} \]
Answer: 10 (Option 1).