A struggle for existence is apparent.
it is not a, because variation is necessary for natural selection
it's not b, because variation is necessary for natural selection
it's not d because natural selection will not occur if the organism is well adapted to its environment.
If a struggle for existence is apparent, natural selection occurs because all of the weak/unfit/unadapted ones die.
the truman doctrine was an american foreign policy whose stated purpose was to counter soviet geopolitical expansion during the cold war. it was announced to congress by president harry s. truman on march 12, 1947, and further developed on july 12, 1948, when he pledged to contain threats to greece and turkey.