Ans. is 'a' i.e., Radial nerve RegionBoundariesContentsUpper triangular space of armSuperior: Teres minorInferior : Teres majorLateral: Long head of tricepsCircumflex scapular arteryLower triangular space of armSuperior: Teres majorMedial: Long head of tricepsLateral; Shaft of humerusRadial nerveProfunda brachii vesselsQuadrangular space of armSuperior: Teres minor, subscapularis, joint capsuleInferior :Teres majorMedial: Long head of tricepsLateral: Surgical neck of humerusAxillary nervePosterior circumflex humeral vessels