Option 1 -> Overwhelm is a synonym of inundate, both meaning to cover or burden excessively.
Option 2 -> Drain means to remove liquid or empty, which is the opposite of flooding or filling.
Option 3 -> Flood is a synonym of inundate, both meaning to cover with water or fill excessively.
Option 4 -> Unyielding means rigid or inflexible, which is unrelated to the concept of flooding.
Hence, Option 2: Drain -> Inundate means to flood or overwhelm with an excessive amount, while drain means to empty or remove liquid, making them direct antonyms -> correct