Five balls numbered 1 to 5 are suspended using separate threads. Pairs (1, 2), (2, 4), and (4, 1) show electrostatic attraction, while pairs (2, 3) and (4, 5) show repulsion. Therefore, ball 1 must be:

Initially, the attraction between Ball 2 and Ball 4 shows that they have opposite charges. Additionally, the attraction between Ball 1 and Ball 2 implies Ball 1 and Ball 4 should share the same charge. However, the attraction between Ball 1 and Ball 4 contradicts this inference. If Ball 2 is positively charged, repulsion between Ball 2 and Ball 3 implies Ball 3 is also positively charged. Similarly, repulsion between Ball 4 and Ball 5 suggests Ball 5 is negatively charged. Ball 1 cannot be positively charged because it attracts Ball 2, nor negatively charged because it attracts Ball 4. Therefore, Ball 1 must be neutral, as it neither repels nor attracts any other balls, indicating no net charge.