Final answer:
Truth is described as "the pure inmate of thy glowing breast" to symbolize its essential role and close relationship with Liberty, meaning Truth is the foundation of Liberty.
Step-by-step explanation:
In the sonnet Sonnet: To Liberty by Mary Robinson, Truth is described as "the pure inmate of thy glowing breast" in line 4 to convey that Truth is closely guarded and inherently resides within Liberty, much like a valued being is kept within one's heart. This metaphor suggests that Liberty nurtures and is intrinsically linked with Truth, implying that without Truth, Liberty would not be as pure or righteous. Hence, the correct answer to why Truth is described this way is to indicate that Truth is foundational to Liberty (option D).