**Core Concept**
Polysaccharides are immunogenic but poorly immunostimulatory in terms of eliciting a strong, protective humoral immune response, especially in children and in the absence of T-cell help. Unlike proteins, they lack the ability to activate T-helper cells, which are essential for robust B-cell activation and class switching.

**Why the Correct Answer is Right**
Polysaccharide antigens are recognized by B cells but do not provide sufficient T-cell help for effective class switching and affinity maturation. This results in weak, short-lived antibody responses, particularly in infants and immunocompromised individuals. In contrast, proteins are highly immunogenic due to their complex structure, which enables strong T-cell-dependent activation and robust antibody production.

**Why Each Wrong Option is Incorrect**
Option B: Protein is highly immunogenic and readily induces strong antibody responses due to T-cell-dependent activation and extensive antigenic diversity.
Option C: "Antigen" is a broad term encompassing all immunogenic substances; the question specifically asks about difficulty in inducing antibody, making this too vague.
Option D: "Effector" is not an antigenic substance and does not trigger antibody production — it refers to immune cells or molecules that mediate immune responses, not a stimulus for antibody induction.

**Clinical Pearl / High-Yield Fact**
Polysaccharide vaccines (e.g., pneumococcal, meningococcal) are less effective in young children and require adjuvants or conjugation to proteins (e.g., in conjugate vaccines) to improve immunogenicity and antibody response.

✓ Correct Answer: A. Polysaccharide