When a kaon decays via the strong interaction the products must include a:

Kaons are mesons made up of a weird quark and either an anti-up or anti-down quark. When a kaon decays via the strong interaction, specific features such as electric charge, baryon number, and strangeness must be preserved. The kaon can decay into a collection of particles that retain these features.

Because kaons have strangeness, a feature that is not conserved in electromagnetic or weak interactions, the decay products must contain at least one weird particle. Weird particles, such as pions or other mesons, have at least one strange quark or antiquark. As a result, (C) weird particle is the correct answer.