What theory did fascists use to support their own ideology

Question

The 4 options are Authoritarianism,Democracy,Communism, and Social Darwinism

Answer 1

If the question is strictly following the early 20th century Fascism (as exemplified by Hitler, Mussolini, Franco, etc), I believe the best answer would be a variation of Social Darwinism & Authoritarianism, if you are able to choose multiple answers. If you must strictly go with one, I would slightly go with SD

1. Democracy = We can exclude Democracy, as Fascist routinely criticize Liberal Democracies as being unable to effectively combat the forces of Marxist-Leninism, as well as being controlled by financial elites who manipulate the people for their own ends. This is famously seen in Hitler's criticism of American-Anglo Finance being controlled by an international Zionist plot (designed to destroy Germany in WWI). That is not to claim Hitler was correct, only that Fascist like him and the greater Nazi party had no interest in maintaining a Democratic form of government. Which is was we see after 1933, when Hitler begins to remove all traces of a liberal democratic order in Germany
2. Communism = We can exclude Communism, as Fascist & Communist saw each other as arch-rivals in geopolitical. From a Fascist perspective, the greatest goal of the state was the preservation & growth of a particular ethnic people (Aryans, Italians, Japanese, etc). In general Marxist thought (at least typified by the USSR in the 1920s), nationalism was what divided the general working class & enabled them to be manipulated by the capitalist class. As such, from a Marxist though, dissolving nationalist ties & emphasizing the commonality of the working class was necessary for the eventual victory over the Bourgeoisie. As a result, Fascist saw Marxist (Communist) working for the exact opposite goal of the Fascist (preserving & expanding a particular nationality).
3. Authoritarianism = As laid out in point #1, the Fascist as seen in Franco's Spain, Imperial Japan, Fascist Italy, and Nazi Germany, had little love towards Democracy. In all four countries, once the Fascist took over, they worked to end a Democratic System & created an authoritarian-totalitarian one party system, under one leader (Il Duce, Emperor, Fuhrer, etc). Central to the Fascist Thesis was that society at large should be organized not just on a economic or political level but on a familiar/social dynamic. Hence why Fascism has tended to "pervade" the non-political spheres of society & desires strict uniformity throughout the "nation-state".
4. Social Darwinism = While I believe #3 is valid, Social Darwinism has also had a long influence on Fascist thought, especially as seen within Nazi Germany & Imperial Japan. Notions of German supremacy had been present since before WWI (the Volkish Movement became rapidly anti-semetic, undoubtedly influencing a young Hitler), but SD gave it a "biological basis" to order the world into a racial hierarchy. SD would be used to justify the eventual enslavement & genocide to multiple people (Slavs, Romanai, and most famously, the Jews), as they were deemed a threat & drag on the Fascist State. The entire notion of Lebensraum rested on the eventual "liquiditation" (i.e. murder) of the people living in the USSR, whereas in Asia, Imperial Japan was creating its own racial hierarchy to justify its imperialism in China & later South-East Asia.

As a result, I would tend to give SD the slight edge over Authoritarianism in Fascist thought. I would see SD & hyper nationalism as the foundation upon which an authoritarian government would be needed to implement the long-term goals of a Fascist State (like a horse before a cart). Hope that helps

Regards,