Choose the word that can substitute the given group of words.

A substance for killing algae

Algaecide

The word that can substitute the given group of words is algaecide.

• Algaecide is a substance that kills algae.
• Prepuce is the skin covering the tip of the penis.
• Fungicide is a substance that kills fungi.
• Ruthenium is a chemical element with the symbol Ru and atomic number 44.

Algaecide is the most appropriate word for the given group of words because it specifically refers to a substance that kills algae. It is derived from the Latin word alga, meaning "seaweed," and the Latin suffix -cide, meaning "killer."