Question

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z line
h line
a line
m line
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Explanation:

The Z - line is a structure in muscle fibers. In the given diagram of a muscle fiber (myofibril), the Z - line is the boundary of a sarcomere. It is a thin, dark line that anchors the thin filaments (actin). The H - line is a lighter area in the middle of the A - band (where only thick filaments - myosin are present). The A - line (A - band) is the region where thick filaments (myosin) are present (including the overlap region with thin filaments). The M - line is in the middle of the H - line and helps in holding the thick filaments together. Based on the structure in the diagram (the thin line that forms the boundary of the repeating unit - sarcomere), it is the Z - line.

Answer:

A. Z line