Why was the cavalry's refusal to fire so serious for Tsarism?

Correct answer: Because the Tsar's repressive order could not be carried out

The governing concept is the dependence of autocratic power on obedience by armed forces. Tsarism could issue orders, but it could survive only if soldiers carried them out, especially when confronting crowds in the capital. The cavalry’s refusal to fire meant that a direct instrument of repression had failed at a critical moment. It also suggested that military loyalty was weakening and encouraged protesters to continue. Option A is correct because the order could not be enforced. Options B, C and D are unrelated: the refusal did not stop foreign trade, raise a peasant tax through the Duma or begin the war.