Answer:
C. Someone opened the window because he or she was too warm.
Step-by-step explanation:
The more recent grammatical usage marking the explicit inclusion of the feminine in the use of pronouns, not the old implicit inclusion where we always assumed the masculine included the feminine.
So, we need to use "he or she" to match with the number expressed in the verb (singular). If we would use 'they', that pronoun is plural. The answer D would have been correct in the past, not nowadays.