Final answer:
The correct word to complete the sentence is 'attended' as it matches the present perfect tense needed for the sentence construction.
Step-by-step explanation:
To answer the question, the correct word to complete the sentence 'I have .... six courses' is 'attended'. This is because the sentence requires a verb in the past participle to match the present perfect tense, which is formed with 'have' followed by a past participle. The present perfect tense describes an action that has been completed at some point in the past relative to the present. 'Attends' is the present tense form and 'attending' is the present participle, used for continuous tenses, neither of which are correct in this context.