Final answer:
The intercalated disk is the unique structure that joins cardiac muscle cells together, facilitating synchronized contractions of the heart through desmosomes and gap junctions. The correct answer is 4) Intercalated disk.
Step-by-step explanation:
Cardiac muscle cells are joined end to end by a unique junctional specialization known as the intercalated disk. The intercalated disk is critical for the support of synchronized contraction of the heart muscle. It consists of desmosomes and gap junctions, which permit the passage of ions between cells, thereby helping to synchronize contraction. Intercellular connective tissue also contributes to binding cells together. The forces exerted by contraction necessitate a strong binding of these cells, which is facilitated by the intercalated discs. Hence, the correct answer to the question is 4) Intercalated disk.