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

Given balls (1)(2)(3)(4)(5). Among them $(1,2),(2,4) \mathrm{and} (4,1)$ shows attraction. Therefore these pairs have either opposite charge or one of them has no charge. Also given $(2,3) \mathrm{and} (4,5)$ show repulsion. 

(2,3) repel each other, (3,4) attract each other and (4,5) repel each other.

$\Rightarrow$(2,3,4,5) are charged , (2,3) are of same nature and (4,5) are of same nature, but (2 and 3) are of opposite nature with respect to ( 2 and 5)

Again, (1) is attracted towards both (2) as well as (4) , hence, (1) is neutral in nature and as it is induced by both the charges , it is made up of metal.