Question

which structure is labeled \a\?
myofibril
sarcomere
sarcoplasmic reticulum
myofilament
sarcolemma

Explanation:

The structure labeled "A" is the tubular network surrounding myofibrils in muscle cells, which matches the definition of the sarcoplasmic reticulum. Sarcomeres are the contractile units within myofibrils, myofilaments are the protein strands making up myofibrils, and the sarcolemma is the muscle cell membrane, none of which match the structure shown.

Answer:

Sarcoplasmic reticulum