An insurer operating in the U.

Detailed Answer in Step-by-Step Solution:

An alien insurer (B) is headquartered outside the U.S. but licensed to operate within it, distinguishing it from a foreign insurer (A), which is domiciled in another U.S. state.

A captive insurer (C) insures its parent company, and a reciprocal exchange (D) is a mutual insurance structure, neither based on location.

The Virginia study guide defines an alien insurer as one incorporated outside the U.S., operating under state licensing, per standard insurance terminology. Reference: Virginia Life, Annuities, and Health Insurance study guide, section on "Insurance Company Types."