The component of blood that prevents its clotting in intact blood vessels is

Concept: Blood does not clot inside healthy, intact vessels because the vessel wall and several natural regulators actively keep the coagulation cascade switched off: the endothelium itself (via surface heparan sulfate and the membrane-bound thrombomodulin–protein C system) plus circulating inhibitors such as antithrombin III and tissue factor pathway inhibitor. Heparin — a sulfated glycosaminoglycan stored in mast-cell granules — is the substance conventionally identified as ‘the anticoagulant component of blood’: once present, it binds antithrombin III and greatly potentiates its ability to inactivate thrombin and the other clotting factors, which is exactly why it is used medically to prevent clotting. Clotting itself is triggered only when a vessel is injured, overriding this restraint.

Application: Applying this to the given options, the component whose defining biological role is anticoagulant — preventing clot formation by potentiating antithrombin III against the clotting factors — is Heparin; among the substances offered, it is the one that fits this role.

Cross-check / Contrast: Checking each of the other components against this same principle confirms none of them fits the anticoagulant role:

• Thrombin's role is to convert fibrinogen into fibrin, driving clot formation — the opposite of the anticoagulant role asked for here.

• Haemoglobin's role is to transport oxygen and carbon dioxide in red blood cells — unrelated to the anticoagulant role asked for here.

• An antibody's role is to identify and neutralize pathogens as part of immune defense — unrelated to the anticoagulant role asked for here.

Since only Heparin's defining role is anticoagulant, it is the answer.