Step 1: Idea: look for a technique with two sides
The question wants two effects together: less of one behaviour and more of another. The technique must act on both.
Step 2: Test option (1)
Token economy strengthens wanted acts with tokens. It acts on one side. Not the best fit.
Step 3: Test option (2)
Modelling teaches through observation. It has no two sided rule. Wrong.
Step 4: Test option (3)
Reciprocal inhibition is about fear and relaxation. Wrong.
Step 5: Test option (4)
Differential reinforcement rewards the good and removes reward for the bad. It acts on both sides. Right.
Step 6: Conclusion
Abby would get praise when he does homework and obeys, and no reward when he annoys others. Over time good conduct rises and bad conduct falls together.
Final Answer:
Option (4), differential reinforcement, is correct.
\[ \boxed{\text{Option (4)}} \]