Concept:Trade unions usually interfere to secure higher wages for their members.Explanation:A trade union negotiates for a wage rate above the equilibrium wage in the labour market.At this higher wage rate, the supply of labour increases because more workers are willing to work.However, employers demand less labour because the cost of hiring workers has risen.The main outcome of this interference is that wages increase, but employment levels are reduced.Answer:D. increased but employment levels are reduced