'Last week' indicates an action that has already completed. This is in simple past tense. 
"Hosted" in option A is the simple past participle of the verb 'host'. This is in tense agreement with the given context.
"Has hosted" indicates present perfect tense and this does not agree with the given context.
"Host" in option B is the plural form of the present participle of the verb. This is incorrect.
"Hosting" is the continuous form of the verb. As the action was completed last week. This is incorrect.
Thus, option A is the correct answer.