Which invention by James Watt significantly improved steam engine efficiency?

James Watt's invention of the condensing engine was a groundbreaking advancement that significantly improved the efficiency of steam engines. Prior to Watt's design, steam engines often wasted a large amount of steam and energy because they could not efficiently utilize the energy produced by condensation.

The condensing engine introduced a separate condenser, which allowed steam to expand in a separate chamber. As the steam was allowed to condense, it created a vacuum that reduced the pressure in the cylinder. This reduction in pressure meant that the steam could do more work with less energy, greatly enhancing overall efficiency. By preventing heat loss and making full use of the steam's energy, the condensing engine enabled engines to operate more effectively and with greater power, transforming industrial processes and transportation during the Industrial Revolution.

The other options do not hold the same significance in relation to Watt's improvements. The rotary engine, while also an innovation, is not directly attributed to James Watt's contributions. The vacuum engine, though intriguing, does not pertain to Watt’s advancements in steam technology. Lastly, the internal combustion engine emerged later and is not relevant to Watt's work on steam engines.