Step 1: Define the core concept
A sentinel is a watcher/protector. Relationship: agent (person) \(⇒\) primary function (verb).
Step 2: Assess choices
(a) Artist : Paint — Agent \(⇒\) action, but "paint" is the method/action of art; not a primary protective function like "watch." Partially similar, not ideal.
(b) Soldier : Battle — Agent \(⇒\) event; "battle" is an incident, not a soldier’s constant duty. The link to function is weak.
(c) Guard : Protect — Agent \(⇒\) primary function. This matches sentinel : watch perfectly. \(⇒\) Best match.
(d) None of these — Incorrect, as (c) provides a perfect match.
Step 3: Summary
Option (c) directly reflects the agent \(⇒\) core function relationship.
\[\boxed{\text{Answer = (c) Guard : Protect}}\]