How can I define lines so when I click on the screen, my program knows whether I clicked on A B or C?
Ask the main part of your question in the title. This should be concise but informative.
Provide everything up front. Don't make people fish for more details in the comments. Provide background information and examples.
Be present for follow up questions. Don't ask for help and run away. Stick around to answer questions and provide more details.
Ask about the problem you're trying to solve. Don't focus too much on debugging your exact solution, as you may be going down the wrong path. Include as much information as you can about what you ultimately are trying to achieve. See more on this here: https://xyproblem.info/
Icon base by Delapouite under CC BY 3.0 with modifications to add a gradient
Sort the shapes front to back. For each shape you check whether the point is inside or not:
Check for intersections between each edge of the shape, and an edge running from the point you're testing to infinity in any direction. For example 10000 units along the x axis. If the total number of intersections is odd, then the point is inside the shape.