Final answer:
D) ClO⁻ is the correct answer as it is the only option listed that will act as a weak base in water by accepting a hydrogen ion from water.
Step-by-step explanation:
The correct answer to which ion will act as a weak base in water is D) ClO⁻. This is because a weak base is defined as a substance that accepts a hydrogen ion (H+) from water, resulting in a slightly ionized solution. In contrast, compound A) OH⁻ is a strong base, B) Cl⁻ is the conjugate base of a strong acid (HCl) and has no basic character, and C) NO₃⁻ is the conjugate base of a strong acid (HNO₃) which also has no basic character. Hence, only D) ClO⁻, the hypochlorite ion, will act as a weak base in water, undergoing a reaction with water to produce OH⁻ and HClO.