Area C is where the gap junctions between cardiac myocytes are. Gap Junctions are found on the plasma membrane of the cardiac myocyte.
The question is basically asking where the plasma membrane is with a bunch of biochem mumbojumbo you dont have to understand.
I thought this image was helpful.
This link has a labeled electron microscope image of cardiac myocytes like the question next to a cartoon image, so it's really helpful. There is also a good description. Scroll all the way down to the last figure.
Actinin is a microfilament protein. α-Actinin is necessary for the attachment of actin filaments to the Z-lines in skeletal muscle cells, and to the dense bodies in smooth muscle cells. The functional protein is an anti-parallel dimer, which cross-links the thin filaments in adjacent sarcomeres, and therefore coordinates contractions between sarcomeres in the horizontal axis.
The non-sarcomeric α-actinins, encoded by ACTN1 and ACTN4, are widely expressed. ACTN2 expression is found in both cardiac and skeletal muscle, whereas ACTN3 is limited to the latter. Both ends of the rod-shaped α-actinin dimer contain actin-binding domains.
Mutations in ACTN4 can cause the kidney disease focal segmental glomerulosclerosis (FSGS).
Myofibrils are composed of smaller structures called myofilaments. There are two main types of filaments: thick filaments and thin filaments; each has different compositions and locations. Thick filaments occur only in the A band of a myofibril. Thin filaments attach to a protein in the Z disc called alpha-actinin and occur across the entire length of the I band and partway into the A band.
The area labeled C is the spot desmosomes between two myocytes and where actin filaments insert on the sarcolemma