The correct answer is B) He means that his inauguration is a celebration of America’s democratic right to elect its leaders rather than the Democrats' victory in the presidential election.
We are referring to the inaugural speech of President John F. Kennedy in January 1961.
In the very beginning of his speech he expressed that sentenced "...we observe today not a victory of party, but a celebration of freedom—symbolizing an end, as well as a beginning—signifying renewal, as well as change."
President Kennedy was smart enough to publicly declared that more than a victory for his political party, that was an occasion to celebrate new beginnings, new opportunities for the country under new leadership. That it was not a victory for the Democrats, but a victory for the United States.
The other complete options of the question were A) He means that his inauguration is not a reason to celebrate with a party-like atmosphere but to feel fortunate we have freedoms, B) He means that his inauguration is a celebration of America’s democratic right to elect its leaders rather than the Democrats' victory in the presidential election, and C) He means that his inauguration is not a reason to reject America’s two-party system but to feel lucky that we live in a free country.