Price wars are 'prima facie; evidence of:

a) Monopoly.

b) Oligopoly.

c) Imperfect competition.

d) Perfect competition.

e) Public good.

Option b) Oligopoly is correct.

The reason is that in oligopolies people have some pricing power and they try to use it by estimating the strategies of their competitors. If their competitors decrease them, they decrease them too and such acts lead to price war.