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

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.