The correct answer is option 3, which is 10 percent reduction in projected demand of conventional energy. The `development of solar cities` program aims to promote the use of solar energy in cities to reduce reliance on conventional energy sources. The program is designed to increase the use of solar energy in various sectors like residential, commercial, and industrial.

Option 1 states a 2 percent reduction in projected demand, which is lower than what the program aims to achieve. Option 2 states a 5 percent reduction, which is also lower than the program`s target. Option 4 states a 15 percent reduction, which is higher than the program`s target.

Therefore, option 3, a 10 percent reduction, is the correct answer according to the `development of solar cities` program. This reduction in projected demand of conventional energy at the end of five years signifies a significant shift towards the use of sustainable and renewable energy sources like solar power.