Step 1: Recall the drill string components: A rotary drill string bottom hole assembly typically has, from the bit upward, the bit, near bit stabilizer, drill collars, more stabilizers, and then drill pipe. The stabilizers are placed at calculated points in this assembly.
Step 2: Purpose from a directional control view: Stabilizer placement controls whether the assembly builds, holds or drops angle, and it keeps the heavy collars away from the borehole wall so the collars do not bend, whip or get differentially stuck.
Step 3: Eliminate distractors related to casing: Statements about casing being equidistant from the wellbore wall describe casing centralizers, a completely separate tool used only when running casing strings, not during rotary drilling with a drill string.
Step 4: Eliminate distractors related to fluid isolation: Isolating wellbore fluid from the formation is achieved through casing and cementing operations, not through the drill string hardware, so this cannot describe stabilizer subs.
Step 5: Confirm the correct role: Since stabilizer subs are rigid, blade-fitted subs installed around the drill collars, their confirmed function is to keep the collars centralized in the hole, giving controlled weight transfer to the bit and a straighter, more stable wellbore.
Final Answer: $oxed{ ext{(C) To assist in keeping the drill collars centralized}}$