Correct Answer: Algaecide

Solution : The correct choice is the first option.

The word that can substitute for the given group of words "a substance for killing algae" is "algaecide."

Algaecide: This is a substance specifically designed for killing or controlling the growth of algae.

The meanings of the other options are as follows:

Prepuce: This term refers to the foreskin, particularly in the context of the male anatomy. It is not related to substances that kill algae.

Fungicide: This is a substance used to kill or inhibit the growth of fungi. While similar in structure to "algaecide," it specifically targets fungi rather than algae.

Ruthenium: This is a chemical element (a transition metal) with the symbol Ru and atomic number 44. It does not relate to substances used for killing algae.

Therefore, the word that can substitute for the given group of words "a substance for killing algae" is algaecide.