Answer:
The lines that use iambic trimeter are B. The first foe in the field and D. A sword, a horse, a shield.
Step-by-step explanation:
This is a poem that alternates the use of tetrameter and trimeter in most of its lines. Iambic trimeter is a meter used in poetry, that as the word says contains three iambics, an iamb is a metrical foot used in poetry that consists of a short syllable followed by a long syllable. iambs also play with the stress where the first is an unstressed syllable followed by a stressed one.