Which element of style does the author use to build the grim mood of this passage? a. word choice b. sentence structure c. an ironic tone d. interior monologue

This depends entirely on the passage. It is impossible to tell from here, but what I can tell you are characteristics you will notice:
if A: words will be used that might describe harmless things, like smiles, but will make comparisons to something unsettling, or bleak, given that the tone is grim. Smiles, for example, might be described as gritted, fake, or spiteful. Usually happy things might be thought of as hollow or fake. 
if B: sentences will be phrased to put emphasis on the worst part of the sentence. Unlikely to be B. 
if C. Ironic tone can mean he is describing possibly extravagant or happy things, and they come off to you as grim. It means he will set up situations that are expected to go well, and will end bleakly. It's unlikely to be this, either, but possible.
if D. internal monologues mean a constant running thought process from the narrator. He will describe his thoughts, and they will be dreary, woeful, and overtake the story as opposed to action or dialogue. Interior monologue means a running narration, and if it builds a grim mood, it will be very bleak and unhappy. It's most likely this, but just based on odds. Only you have read the passage. 

There exists a similar question wherein the passage was given. The passage that needs to be internalized is an internal monologue. Thus, the answer to this question is letter D. This is the way that the author allowed the inner thoughts and mood of the speaker come out.