The sliding filament theory explains muscle contraction at the level of:

Correct Answer: B

Rationale: The sliding filament theory describes how muscle contraction occurs when actin and myosin filaments slide past each other, shortening the muscle. This process happens within the sarcomere, the functional unit of contraction between Z-lines in a myofibril. While myofibrils contain sarcomeres and myosin heads facilitate the sliding, the theory specifically applies to the sarcomere level, making 'Sarcomeres' the correct answer.

Correct Answer: B

Rationale: Compact bone, also called cortical bone, is the dense, hard outer layer of bone tissue that provides strength and structural support. Spongy bone (with trabeculae) is lighter and found inside, often in the epiphysis, but 'compact bone' specifically denotes the dense type, making 'Dense and hard' the correct answer.

Correct Answer: D

Rationale: The neuromuscular junction is the synapse where a motor neuron contacts a muscle fiber, releasing acetylcholine to initiate contraction. Sarcoplasm is muscle cytoplasm, myofibrils are contractile components, and sarcomeres are within myofibrils, making 'Neuromuscular junction' the correct answer.

Correct Answer: C

Rationale: The sarcoplasmic reticulum, a specialized endoplasmic reticulum in muscle cells, stores calcium ions and releases them into the cytoplasm upon a nerve signal. This calcium triggers contraction by binding to troponin. It doesn’t produce ATP or contract myofibrils, making 'Storing and releasing calcium ions' correct.

Correct Answer: B

Rationale: The radius and ulna, forearm bones, are part of the appendicular skeleton, which includes the limbs and girdles. They are long bones but belong to the appendicular division, not axial. 'Long bones' is a type, not a division, making 'Appendicular skeleton' correct.