Answer: B. Well-trained horses
Step-by-step explanation:
The narrator in this excerpt spoke of how the horses helped the Britons fight effectively in battle. These horses were so well trained that even though this excerpt is based in the past, the training the horses had then is more or less the same as now which indicates a lack of a need or capacity to improve.
The horses would carry their masters into battle and go straight into danger without fear because they were trained to do so. This enabled the Britons to fight on chariots from which they could deal devastating blows to the enemy.