The chemical compound that stops the production of proteins will directly affect the ribosomes in the tadpoles' cells. Ribosomes are the cellular structures responsible for synthesizing proteins by translating messenger RNA (mRNA). If the compound inhibits protein production, it is likely acting on the ribosomes or the process of translation.
Vacuoles, chloroplasts, and mitochondria are involved in other cellular processes (storage, photosynthesis, and energy production, respectively) and are not directly responsible for protein synthesis.