Option 1: will be spending -> Future continuous tense indicates an ongoing action, but doesn't convey completion by a specific time.
Option 2: will have spent -> Future perfect tense correctly indicates an action that will be completed by a specific point in the future ("by the end of the month").
Option 3: shall spend -> Simple future tense doesn't emphasize the completion of spending all money by a deadline.
Option 4: will spend -> Simple future tense suggests the action will happen but doesn't convey that it will be completed by the specified time.
Hence, Option 2: will have spent -> The phrase "by the end of the month" requires future perfect tense (will have + past participle) to indicate that the action of spending will be completed before that specific future time, resulting in all his money being exhausted -> correct