Which one of the following factors constitutes the best safeguard of liberty in a liberal democracy?

(a) A committed judiciary — Not the best safeguard. In a liberal democracy, judiciary must be independent and impartial, not "committed" to an ideology or government.
(b) Centralization of powers — Incorrect. Concentration of power threatens liberty; liberal democracy prefers limited government and checks.
(c) Elected government — Necessary but not sufficient. An elected government can still become majoritarian or arbitrary without institutional restraints.
(d) Separation of powers — Correct. Liberty is best protected when legislature, executive, and judiciary check each other, preventing abuse of power.