Final answer:
Option (a) Mg(OH)₂ is considered a base as it contains the hydroxide ion (OH⁻), characteristic of bases. The other options, C₆H₅COOH and CH₃COOH, are acids and do not qualify as bases.
Step-by-step explanation:
In the question, we are asked to identify which of the given substances is a base. Option (a) Mg(OH)₂ is the correct answer because it contains the hydroxide ion (OH⁻). The presence of the hydroxide ion in compounds, like NaOH and Ca(OH)2, is a characteristic feature of bases, and these compounds can accept a proton from acids to form water. Magnesium hydroxide, Mg(OH)2 is listed in various tables as a strong base, although it should be noted that in actuality, its solubility is quite low, making it less dissociated in water than other strong bases.
Options (b) and (c), C₆H₅COOH and CH₃COOH, are both acids, with the former being benzoic acid and the latter acetic acid, neither of which are bases. In general, acid compounds donate protons in reactions, as opposed to bases which accept protons.