How should you complete the sentence: I am looking forward to ________ my birthday party at work?

The correct completion of the sentence is "I am looking forward to having my birthday party at work." The phrase "looking forward to" is commonly followed by a gerund, which is the present participle form of a verb, ending in "-ing." In this case, "having" serves as the gerund, indicating anticipation of the event (the birthday party).

Using "have," "has," or "had" would create grammatical errors since they do not align with the structure prompted by "looking forward to." "Have" is the base form of the verb, while "has" is the third person singular form, and "had" indicates a past event. None of these forms fit seamlessly into the context of expressing future anticipation regarding an event. Therefore, the use of "having" is the appropriate choice to convey the intended meaning correctly.