Arduino Line Follower/Object Detector

I have to make a line follower robot that detects an object in each of the layouts given in the image attached. Each section labelled 1,2,3,4,5,6 has people (hard tennis balls) indicated by the green blobs. The line follower must be able to detect people from outside and a green LED must flash every time for every person detected. The diameter of the ball is 80 mm and the wall of the compartments are 25 mm.
The order of detection must be 1,2,3,4,5,6.

So for those who thought tl;dr--

1. Which type of sensors can I use to in order to detect the balls? ( I presume it is easier to detect from the intersection point of the black lines,since it will be easier to move around.)
2. How can I set up the IR sensor array on the bot such that it detects the balls and turns accordingly at the intersection to move around?

PS: The bot MUST BE AUTONOMOUS