Identify parts of speech of the underlined word: 

The correct preposition to fill in the blank is "by". The phrase "by the time" is commonly used with the future perfect tense to indicate that an action will be completed at or before a specific point in the future.

    
• Future Perfect Tense: The sentence "I'll have finished" uses the future perfect tense (will + have + past participle), which describes an action that will be completed before a certain time in the future.
    
• "By the time": This phrase introduces a time clause that specifies the deadline or the point in the future when the main action (finishing) will be completed. It means "not later than" or "when that particular point in time arrives."
    
• Example: "I will have finished my report by the time my boss arrives." Here, "by the time my boss arrives" sets the deadline for completing the report.

As soon as (যত তাড়াতাড়ি সম্ভব): The tree began to decay as soon as it was cut down.

As through (যেন): He behaves as though he were he owner of the house.