Long bones function in
A. Support
B. Support, erythrocyte, and leukocyte synthesis
C. Support and erythrocyte synthesis
D. Erythrocyte formation

Hint: Long bones are mostly located in the appendicular skeleton. It includes bones in the lower limbs (the tibia, fibula, femur, metatarsals, and phalanges) and bones in the upper limbs (the humerus, radius, ulna, metacarpals, and phalanges).

Complete answer:
The long bones are those that are longer than they are wide. Long bones, especially the femur and tibia, are subjected to most of the load during daily activities and they are crucial for skeletal mobility. Up to the age of 20 years, RBCs are produced from the red bone marrow of all the bones (long bones and all the flat bones).
After the age of 20 years, RBCs are produced from membranous bones such as vertebrae, the sternum, ribs, scapulas, and iliac bones.
So, the correct answer is B, i.e., Support, erythrocyte, and leukocyte synthesis

Additional information:

The end of the long bone is the epiphysis and the shaft is the diaphysis. When humans finish growing these parts fuse together. They grow primarily by elongation of the diaphysis, with an epiphysis at each end of the growing bone. The ends of epiphyses are covered with hyaline cartilage ("articular cartilage"). The longitudinal growth of long bones is a result of endochondral ossification at the epiphyseal plate.
Bone growth in length is stimulated by the production of growth hormone (GH), a secretion of the anterior lobe of the pituitary gland. The outside of the bone consists of a layer of connective tissue called the periosteum. Additionally, the outer shell of the long bone is compact bone, then a deeper layer of cancellous bone (spongy bone) which contains in the medullary cavity the bone marrow.

Note: There are five types of bones: long, short, flat, irregular, and sesamoid. After 20 years of age, the shaft of the long bones becomes yellow bone marrow because of fat deposition and loses the erythropoietic function.