The correct option is (a)  the desire for a constitutional monarchy. 
The desire for a constitutional monarchy was the cause of the Decembrist revolt.
What was the Decembrist revolt?
- Before the Russian revolution, Decembrist revolt was the revolt to overthrow the Russian Empire. 
- The Decembrist leaders wanted the Russian government to not be in power. They wanted to remove the tsar’s family from power. 
- The Decembrists were the people of the Russian military and noblemen. They were also the members of multiple anti-government societies that were formed in secret. 
- These people together organized the Decembrist revolt of 1825. 
- Some royal guards Alexander Muravyov and Nikita Muravyov were also involved in this. 
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