Option 1 -> Alleviate means to reduce, lessen or make less severe, which aligns with mitigate.
Option 2 -> Strengthen means to make more powerful or robust, which is opposite to mitigate.
Option 3 -> Aggravate means to make worse or more serious, which is the opposite of mitigate.
Option 4 -> Intensify means to increase in severity or strength, which contradicts mitigate.
Hence, Option 1: Alleviate -> In the passage context, the Future of Life Institute works to "mitigate existential risk facing humanity," meaning they aim to reduce or lessen these risks. Mitigate and alleviate are synonyms, both referring to making something less severe or harmful. -> correct