Gas turbines are widely used in power plants to generate electricity. They are known for their high efficiency and ability to quickly respond to changes in power demand. Reheaters are used in gas turbine plants to further enhance their performance. Let's discuss the two statements given in the question and understand why option 'A' is the correct answer.

1. Considerable improvement in the work output:
- Reheaters are heat exchangers that are placed between the high-pressure and low-pressure stages of a gas turbine. They are used to extract additional energy from the exhaust gases before they enter the low-pressure turbine.
- The exhaust gases leaving the high-pressure turbine are at a high temperature, and they still have a significant amount of energy. By passing them through a reheater, this energy can be utilized to further expand the gases and generate additional work.
- The reheater increases the mass flow rate of the gas turbine, which leads to a higher overall work output. This means that more electricity can be generated by the gas turbine plant.
- Therefore, the statement that a gas turbine plant with a reheater leads to a considerable improvement in the work output is correct.

2. Considerable improvement in the thermal efficiency:
- Thermal efficiency is a measure of how effectively a power plant converts the heat energy into useful work. In gas turbine plants, the thermal efficiency is determined by the temperature difference between the combustion gases and the ambient air.
- By using a reheater, the temperature of the combustion gases leaving the high-pressure turbine can be reduced. This reduces the temperature difference between the combustion gases and the ambient air and improves the thermal efficiency.
- The reheater also allows for a higher pressure ratio across the turbine, which further improves the thermal efficiency of the gas turbine plant.
- Therefore, the statement that a gas turbine plant with a reheater leads to a considerable improvement in the thermal efficiency is correct.

In conclusion, both statements given in the question are correct. A gas turbine plant with a reheater leads to a considerable improvement in the work output and the thermal efficiency. Reheaters are an important component of gas turbine plants that enhance their performance by extracting additional energy from the exhaust gases and reducing the temperature difference between the combustion gases and the ambient air.