Which of the following is an example of nationalism?

A. A leader calls on his country's allies to protect his country from a
threat posed by neighboring nation-state.

B. Under an alliance, one country promises to defend another
country from an attack by a third.

C. An activist organizes a protest of ethnic minorities in a country to
demand that they be given their own country.

D. One country declares war on another power after that power
invades the first country's ally.