Dari is the official and the most common language spoken in Afghanistan.
CIA World Factbook are listed in the chart in the sidebar, below the map of languages by region. Persian (Dari dialects) 50% and Pashto 35%. Pashto and Persian are the official languages of the country. Hazaragi, spoken by the Hazara minority, is another dialect of Persian. Other languages spoken include Turkic languages (primarily Uzbek and Turkmen) 9%, as well as 30 minor languages 4% (primarily Balochi, Nuristani, Pashai, Brahui, Pamiri languages, Hindko, etc.). Bilingualism is common.
According to the Encyclopædia Iranica, the Persian language is the mother tongue of roughly one-third of Afghanistan's population, while it is also the most widely used language of the country, spoken by around 80% of the population. It further states that Pashto is spoken by around 50% of the population.
Translation: Dari (Farsi) is the language of majority of Afghanistan's people.