Which of these is one way that a thesaurus differs from a dictionary?

Unlike the dictionary, the words in a thesaurus are arranged either alphabetically (just like the dictionary) or thematically (words of similar meanings grouped together). While a dictionary defines a word, a thesaurus gives a choice of words for each entry.

Don't know the context to your question, but that may because there is none. 
I would go with a). unique; diverse 

We are looking for a choice that will apply to ALL audiences. With that defense
b). probably would not work because not all audiences are attentive, but you hope that they will be. 
c). Doesn't sound right...individuals are for single people, and the way the question is phrased makes the audience a whole. 
Could Say: All audience members are individuals
d). groups; subgroups may work, but an audience is usually categorized as a whole group.