Final answer:
The heart (c) is not primarily composed of connective tissue but rather of cardiac muscle tissue, in contrast to bone marrow, articular cartilage, and mesenchyme which are mainly connective tissues.
Step-by-step explanation:
The option that is NOT primarily composed of connective tissue is c. Heart. While bone marrow (a), articular cartilage (b), and mesenchyme (d) are indeed primarily made up of connective tissue, the heart is primarily composed of muscle tissue, specifically cardiac muscle tissue. Connective tissue plays a role in the structure and function of the heart, for instance, the heart valves and the pericardium, but it is not the principal tissue type. Bone tissue is described as dense, hard connective tissue, and blood, cartilage, and bone are considered connective tissue but distinctly different from 'connective tissue proper'. Mesenchyme is an embryonic connective tissue giving rise to various structures including connective tissues, and during embryonic development, bones and joints develop from mesenchyme through processes like intramembranous and endochondral ossification.