Final answer:
KOH (potassium hydroxide) is the only compound among the given options that qualifies as a strong base, as it ionizes completely in an aqueous solution.
Step-by-step explanation:
The question asks which of the listed compounds is a strong base. Among the options given:
A) HCl is a strong acid.
B) KOH (potassium hydroxide) is a strong base, as it ionizes completely in an aqueous solution.
C) CaCO3 (calcium carbonate) is not a base but a salt that can act as a weak base or acid depending on the situation.
D) AgNO3 (silver nitrate) is neither a base nor an acid; it is a salt.
Based on this information, the compound that is a strong base is B) KOH.