Answer: It is the trickiest door to open.
Step-by-step explanation:
We can infer from the text that the author here believes that the voice of a character is very challenging to hear from the phrase about it being the trickiest door to open and the hardest to close. This is therefore the author's opinion about voice.
The second option could not be the answer because they used anima not voice. The third and fourth options are incorrect as well because the writer can hear the voice but the reader does not always hear the voice as they only hear it if the writer does.