Final answer:
The line given is a simile because it uses the word 'as' to make a direct comparison between the speaker's thoughts and things that are not readily understood, like dreams and birds' speech.
Step-by-step explanation:
The quoted text suggests a sense of thoughts being incomprehensible or meaningless to others, similarly to how dreams can be confusing or how birds' speech is unintelligible to humans. Four literary devices are presented as options: simile, metaphor, hyperbole, and personification. Considering the comparison made using 'as' in the text, the line "My thoughts are senseless to mankind As a dream's trouble or the speech of birds." is an example of a simile because it directly compares the speaker's thoughts to dreams and birds' speech using the word 'as', a key marker of a simile.