Step 1: Read the question.
We must say what a smart classroom mainly means.
Step 2: Idea of a smart classroom.
A smart classroom is a room where digital technology is built into teaching. Its goal is to make learning interactive and learner centred.
Step 3: How it differs from old classrooms.
A normal class uses blackboards, textbooks, and spoken words. A smart class adds modern digital tools to support learning.
Step 4: The key tool.
The main feature is the use of interactive boards, where teachers and students can write, touch, and show content live.
Step 5: Reject the other options.
Physical textbooks, pictures and charts, and green boards are traditional tools. They do not define a smart classroom.
Step 6: Final answer.
A smart classroom means the use of interactive boards.
\[ \boxed{\text{Use of interactive boards.}} \]