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A
Carry oxygen from lungs to body tissues and carbon dioxide back to lungs; contain hemoglobin which binds oxygen
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B
Produce hormones
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C
Fight infection
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D
Form blood clots
Why this is the answer
Red blood cells (RBCs/erythrocytes) — most abundant blood cells (~4-6 million per microliter). Primary function: gas transport. Hemoglobin: iron-containing protein; each hemoglobin molecule binds 4 oxygen molecules; gives RBCs and blood their red color. RBC characteristics: biconcave disc shape (increases surface area, allows flexibility); no nucleus in mature cells (more room for hemoglobin); ~120 day lifespan; produced in bone marrow (stimulated by erythropoietin from kidneys); destroyed in spleen and liver. Blood composition: (1) Plasma (~55%) — water, proteins, nutrients, waste, hormones; (2) Cells (~45%): RBCs (most), white blood cells (immune), platelets (clotting). White blood cells (leukocytes): fight infection and disease; ~5,000-10,000/μL; types include neutrophils (most common, fight bacteria), lymphocytes (T and B cells; antibodies, immune memory), monocytes (become macrophages), eosinophils (parasites, allergies), basophils (allergies). Platelets (thrombocytes): cell fragments; ~150,000-400,000/μL; activate to form clots, stopping bleeding. Anemia: low RBC count or hemoglobin; causes fatigue, weakness, pallor; types include iron deficiency, B12/folate deficiency, blood loss, hemolytic, aplastic. Polycythemia: too many RBCs.
Source: ASVAB Science — Blood Cells