To study the economic condition of the workers of a factory, universe will be

Officials of the trade union
Married employees only
Educated employees only
All labours

The correct answer is D. All labours.

The universe is the population or group of people from which a sample is drawn. In this case, the universe is all the workers of a factory. This is because the study is about the economic condition of all the workers, not just a specific group of workers.

Option A is incorrect because the officials of the trade union are not the only workers in the factory.

Option B is incorrect because married employees are not the only workers in the factory.

Option C is incorrect because educated employees are not the only workers in the factory.