Which of these rhetorical devices was the author of the declaration of independence using when he included words like cruelty and barbarous?

Diction.

Explanation:

Diction is demonstrated as one of the most impactful rhetorical devices that involve the 'choice of words' and 'style of expression' that assists in creating the intended tone and mood in the work that would promote elicitation of desired feelings and impact on the audience.

As per the question, 'diction' is the rhetorical device that has been employed by the authors of 'Declaration of Independence' when the words like 'cruelty and barbarous' were chosen to be included in the document as these words provide the vivid description of the sufferings faced by Americans through which the audience could have a sensory experience and establish an emotional connection to it. Thus, they would be able to elicit the desired response as intended by the authors of the Declaration. Therefore, 'diction' is the correct answer.