Tsar Nicholas was prevailed upon to abdicate when D. troops garrisoned in the capital mutinied.
Tsar Nicholas became deeply unpopular in the early 1900s and this only became worse as the first World War progressed.
The only reason he was still able to rule was because the army supported him. Once the army garrison in the capital mutinied, he knew his time was up and he abdicated.
In conclusion, option D is correct.
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