Why was the slogan all power to the Soviets a challenge to the Provisional Government?

Because it shifted power from government ministers toward people's councils

The correct answer is A. After the February Revolution, the Provisional Government formally held state authority, but workers’, soldiers’, and later peasants’ soviets represented organised popular groups. The slogan “All power to the Soviets” demanded that real political authority move away from the ministers and be transferred to these councils. It therefore challenged both the legitimacy and the survival of the Provisional Government, especially because the government continued the war and did not immediately solve demands for land, food, and peace. The slogan did not support Tsarism, foreign trade, or merely a grain tax. Its central issue was the location of political power and the identity of the institutions entitled to exercise it.

This is a Class 9 History question. Chapter: Socialism in Europe and the Russian Revolution. Topic: The Russian Revolution.