Step 1: Understand the question.
We must find the AI technique used in chatbots like ChatGPT.
Step 2: What a chatbot does.
A chatbot reads what a person types in normal language, understands it, and writes back a sensible reply in the same language.
Step 3: Meet Natural Language Processing.
Natural Language Processing, or NLP, is the AI field that lets computers understand and produce human language.
Step 4: Link NLP to chatbots.
Because a chatbot must read and reply in human words, it relies on NLP to grasp meaning and form answers.
Step 5: Reject the other options.
Image processing handles pictures, data compression shrinks files, and encryption hides data. None of these is about understanding language.
Step 6: Conclude.
So the technique used in chatbots like ChatGPT is Natural Language Processing, which is option 2.
\[ \boxed{\text{Natural Language Processing}} \]