The most lasting contribution of the Rastrakutas is option 2, which includes the Kailasha Temple and the three writers of Kannada poetry - Pampa, Ponna, and Ranna. The Kailasha Temple, located in Ellora, Maharashtra, is an architectural marvel and a symbol of their artistic excellence. It is a monolithic structure carved out of a single rock, showcasing the skill and craftsmanship of the Rastrakutas. Pampa, Ponna, and Ranna were prominent poets of their time and contributed significantly to Kannada literature. Their works are still revered and studied today.
Option 1, the Kailasha Temple by itself, is correct but incomplete as it does not include the contribution of the three writers. Option 3, patronage of Jainism, and option 4, conquests, may have been important aspects of the Rastrakuta dynasty, but they are not mentioned as the most lasting contribution in this context.