Answer: C) Indices.
Explanation: Indexes and indices are both accepted and widely used plurals of the noun index, "Indices" is the original plural (in Latin), but "indexes" is the plural in English. Both of them are correct, but from the given options "indici" and "indexers" aren't accepted plurals for the word "index." So the correct answer is the corresponding to option C: It was so gross out today: the heat and huminity indices were off the charts!
