Step 1: Understand the question.
We must find the activity that best builds thinking and reasoning skills.
Step 2: How thinking grows.
Thinking and reasoning grow when learners actively analyse, compare, discuss, and solve problems, not when they just sit quietly.
Step 3: Look at the passive options.
Listening to lectures, repeating similar exercises, and copying notes keep the learner passive. They build memory more than reasoning.
Step 4: Look at group discussion.
In group discussions to solve problems, students share ideas, defend their views, weigh choices, and reason together.
Step 5: Why it works best.
This active exchange forces learners to think, question, and judge, which is exactly how reasoning skills develop.
Step 6: Final answer.
The best activity is group discussions to solve problems.
\[ \boxed{\text{Group discussions to solve problems.}} \]