Final answer:
Austria and Prussia were the two nations that supported the French royal family during the French Revolution in 1791, reflecting their desire to maintain monarchial order in Europe.
Step-by-step explanation:
The two nations that came to the aid of the French royal family in 1791 during the French Revolution were Austria and Prussia. These countries had a vested interest in suppressing the revolution and maintaining the monarchical status quo in Europe. Even though, eventually, there were significant shifts in alliances and wars such as the involvement of the United States with France based on the 1778 Treaty of Alliance during its own war against Great Britain, and Russia alongside Austria and Prussia forming the Holy Alliance in 1815 to preserve conservative monarchies. The correct answer to the question regarding aid to the French royal family during 1791 is, therefore, D. Austria and Prussia.