Final answer:
The literary device in the provided sentences is a metaphor, which compares two unlike things directly without using 'like' or 'as'. The sentences create a figurative image that prompts a deeper interpretation of silent actions. Therefore, the correct option is a) Metaphor.
Step-by-step explanation:
The literary device present in the sentences "They spoke without a voice" and "They leaped without a sound" is a metaphor. A metaphor is a direct comparison between two unlike things without using the words "like" or "as". The lines suggest that the subjects are performing actions typically associated with noise in a silent or unexpected manner, implying a comparability between them and something that inherently lacks these features. This comparison doesn't use "like" or "as," which would be indicative of a simile.
On the flip side, alliteration is the repetition of consonant sounds at the beginning of words, which is not evident in these lines. Furthermore, hyperbole refers to an exaggerated statement that's not meant to be taken literally, and there's no exaggeration here. Instead, the sentences create a figurative image, invoking readers to uncover the deeper meaning behind the actions being performed silently.