Theophylline is a methylxanthine derivative that acts as a non-selective adenosine receptor antagonist. By blocking adenosine receptors (particularly A1 and A2), it prevents adenosine-mediated effects such as bronchoconstriction, sedation, and vasodilation. This antagonism enhances bronchodilation and increases cerebral blood flow.

Histamine, bradykinin, and imidazoline receptors are not primary targets of theophylline. While it may have weak interactions with other systems, its main pharmacological action is through adenosine receptor blockade.

**✓ Correct: C. Adenosine receptors**