Select the option which best expresses the following sentence in the…

The correct answer is Option (1) → The reply must be sent by you.

Active to passive voice with a modal:

Change	Active	Passive	Why
Object becomes subject	the reply	The reply	“The reply” moves to the front.
Tense kept	must send	must be sent	The modal stays, followed by be + past participle: “must send” becomes “must be sent” (sent is the past participle of send).
Agent (by ...)	You	by you	“You” goes to the end as “by you”.
Full sentence	You must send the reply.	The reply must be sent by you.

Example: You should water the plants. → The plants should be watered by you.

Why the Other Options are Incorrect?

Option 2: The reply must have been send by you. — Incorrect.

• "Must have been" changes the sense to a guess about the past.
• "Send" is the base form, not the past participle "sent".

Option 3: The reply must be send by you. — Incorrect. The pattern is right, but the passive needs the past participle "sent", not "send".

Option 4: The reply must have been sent by you. — Incorrect. "Must have been sent" is a guess about something already done. The active sentence is an order for the present or future.

Contrast: "must be sent" is an order for now or later. "Must have been sent" is a guess about the past.