The correct answer is D) to spread the Revolution and destroy other monarchies.
The Radical Legislative Assembly declared war on Austria and then Prussia in 1792 to spread the Revolution and destroy other monarchies.
At the end of the 1700s, European monarchies were worried by the results of the Frech Revolution. Monarchs were afraid that the revolutionary spirit that inspired the French people would be contagious and something similar could happen in their nations.
In France, the Radical Legislative Assembly that has just been elected wanted to spread the revolution to other countries and destroy the monarchies of Austria and Prussia, and that is when France declared war against them. The Assembly had information that people that left France during the Revolution were hiding in those territories, preparing a counter-attack to the Revolution.