So far we ____ the living room and the kitchen. ?

The correct choice for the sentence "So far we ____ the living room and the kitchen" is C) "have decorated." This selection is appropriate because it employs the present perfect tense, which is used to describe actions that occurred at an unspecified time in the past and are relevant to the present.

In this context, "have decorated" indicates that the action of decorating the living room and the kitchen occurred at some point before now and is significant to the current situation. The phrase "so far" suggests that the decorating is part of an ongoing process, implying that there may be more decorating to do in the future.

The other options do not fit as well:

- A) "decorated" is the simple past tense and fails to connect the action to the present moment.
- B) "decorate" is in the simple present tense and does not reflect a completed action.
- D) "have decorating" is grammatically incorrect as it mixes tenses improperly.

Thus, C) "have decorated" effectively conveys the intended meaning of a completed action that still has relevance now.