Read the passage carefully and answer the questions based on the passage:

Increasing return to scale (IRS) implies that if we increase all the inputs by a certain proportion, output increases by more than that proportion. In other words, to increase output by a certain proportion, inputs need to be increased by less than that proportion. With the input prices given, cost also increases by a lesser proportion. For example, suppose we want to double the output. To do that, inputs need to be increased, but less than double. The cost that the firm incurs to hire those inputs therefore also need to be increased by less than double. What is happening to the average cost here? It must be the case that as long as IRS operates, average cost falls as the firm increases output. Decreasing return to scale (DRS) implies that if we want to increase the output by a certain proportion, inputs need to be increased by more than that proportion. As a result, cost also increases by more than that proportion. So, as long as DRS operates, the average cost must be rising as the firm increases output. Constant return to scale (CRS) implies a proportional increase in inputs resulting in a proportional increase in output. So the average cost remains constant as long as CRS operates. It is argued that in a typical firm IRS is observed at the initial level of production. This is then followed by the CRS and then by the DRS. Accordingly, the LRAC curve is a 'U'-shaped curve. Its downward sloping part corresponds to IRS and upward rising part corresponds to DRS. At the minimum point of the LRAC curve, CRS is observed. 

In which of the following conditions, output increases with the same proportion of input?

Constant return to scale.

The correct answer is Option (3) → Constant return to scale.

When a firm experiences constant return to scale, the output increases in the same proportion as the increase in inputs. This means that if all inputs are doubled, the output also doubles. The passage clearly states that under constant return to scale, there is a proportional increase in inputs resulting in a proportional increase in output, and therefore, the average cost remains constant.