Step 1: Understanding the Problem:
The question asks to identify which segment of the global population is most severely affected by nutritional anaemia (primarily iron-deficiency anaemia).
Step 2: Detailed Explanation:
• Anaemia Definition: Anaemia is a condition characterized by a decrease in the total amount of red blood cells (RBCs) or hemoglobin in the blood, reducing the oxygen-carrying capacity of the blood. It is primarily caused by dietary iron deficiency, folates, or Vitamin $B_{12}$ deficiencies.
• Vulnerable Population Groups: While pre-school children and non-pregnant women of reproductive age also experience high rates of anaemia, pregnant women represent the group with the highest physiological vulnerability.
• Physiological Basis in Pregnancy:
• During pregnancy, maternal blood volume expands by approximately $50\%$ (hemodilution) to support fetal development.
• This expansion dramatically increases the maternal physiological requirement for iron to produce hemoglobin for both the mother and the growing fetus and placenta.
• If dietary iron intake or pre-existing iron stores are insufficient, iron-deficiency anaemia develops rapidly. According to World Health Organization (WHO) reports, globally, over $40\%$ of pregnant women are anaemic.
• Men: Men have lower iron requirements due to the absence of menstrual blood loss and lower physiological demands, making them the least affected group.
Step 3: Final Answer:
Pregnant women are the global population group most severely affected by anaemia due to high maternal-fetal iron demands, corresponding to option (B).