Explanation:
The correct option is 'c'. Pain that is out of proportion to the primary injury and exacerbated by passive stretching of the muscles within the involved compartment is the earliest and most sensitive clinical indicator of acute compartment syndrome. The classic '5 Ps' (Pain, Pallor, Pulselessness, Paresthesia, Paralysis) are often taught, but pallor, pulselessness, and paralysis are very late signs indicating irreversible tissue damage. Pulses are often maintained until the compartment pressures are extremely high, well above the threshold for muscle and nerve ischemia.

Reference: McQueen MM, Court-Brown CM. Compartment monitoring in tibial fractures. The pressure threshold for decompression. J Bone Joint Surg Br. 1996;78(1):99-104.