We are being asked to balance the given half-reaction under basic solution
Step 1. Balance the non-hydrogen and non-oxygen elements first
Step 2. Balance oxygen by adding H2O to the side that needs oxygen (1 O: 1 H2O)
O2(g) →H2O(l) + H2O(l)
Step 3. Balance hydrogen by adding H+ to the side that needs hydrogen (1 H: 1 H+)
4H+(aq) + O2(g) →2 H2O(l)
Complete and balance the half reaction O2(g) →H2O(l) in basic solution.