What lacking factor contributed to the failure of Ivan's solar energy invention?

The correct answer is economic viability, which refers to the ability of a product or idea to be financially sustainable and potentially profitable in the market. In the case of Ivan's solar energy invention, if it lacked economic viability, it suggests that even if the invention was technically sound and environmentally friendly, it could not generate sufficient revenue or profit to justify its production and maintenance costs.

For an invention to be successful in the marketplace, it not only needs to solve a problem or fulfill a need but also has to do so in a way that makes financial sense to both the company and potential customers. If there is insufficient demand or the costs are too high relative to the potential market price, the invention will struggle to attract investment and market adoption.

Technical feasibility, environmental impact, and market demand are all important factors in the success of an invention, but if the economic model does not support the idea, the other factors become moot. Hence, without economic viability, no amount of technical innovation or positive environmental impact will ensure the success of the invention.