Histamine is secreted by

The correct answer is mast cells.

Explanation:
Histamine is a key organic nitrogenous compound involved in local immune responses, inflammatory reactions, and regulating physiological function in the gut and acting as a neurotransmitter.
In connective tissues, histamine is synthesized, stored, and released primarily by mast cells (and also by basophils in the blood).
When mast cells are activated, typically during an allergic reaction or in response to tissue injury, they undergo degranulation, releasing histamine into the surrounding extracellular environment. This leads to physiological effects such as vasodilation (dilation of blood vessels) and increased vascular permeability.

Analyzing the other options:
1. Histiocytes: These are stationary phagocytic cells (macrophages) found in connective tissue. Their primary function is phagocytosis (engulfing debris and pathogens), not secretion of histamine.
2. Lymphocytes: These are white blood cells of the immune system (such as B cells and T cells) responsible for adaptive immunity, producing antibodies or killing infected cells.
3. Fibroblasts: These cells are responsible for synthesizing the extracellular matrix and collagen, providing the structural framework for tissues, rather than secreting inflammatory mediators like histamine.