Final answer:
The literary device used in the phrase to convey unity or closeness is not explicitly present, but the correct answer is alliteration by process of elimination, as simile, metaphor, and hyperbole do not fit the context.
Step-by-step explanation:
In the phrase "In one another's arms. Birds in trees'", the literary device used to convey a sense of unity or closeness is not based on comparison or a play on words, but rather on the scenario described. It doesn’t contain a comparison using 'like' or 'as' (which would indicate a simile), nor does it compare two unlike things directly (which would suggest a metaphor), nor does it have an overstatement or exaggerated claim (which would be hyperbole). Therefore, by elimination, the correct answer is A. Alliteration. Alliteration is the repetition of initial consonant sounds, which is not explicitly present in the phrase. However, since none of the other options fit, we can infer that the question intends to emphasize the presence of unity or closeness without the need of those specific literary devices.