Select the option that expresses the given sentence in passive voice.…

The correct answer is Option (1) → The cows used to be milked by my grandfather every morning.

Passive of “used to” + verb:

Change	Active	Passive	Why
Object becomes subject	the cows	The cows	The cows receive the action, so they become the subject.
Used to kept	used to milk	used to be milked	“Used to milk” tells of a past habit. In the passive it becomes used to be + past participle (V3). “Used to” does not change for one or many.
Agent (by ...)	My grandfather	by my grandfather	My grandfather is named, so the doer comes after “by”.
Full sentence	My grandfather used to milk the cows every morning.	The cows used to be milked by my grandfather every morning.

Other verbs after “used to” work the same way. Example: The villagers used to draw water from this well. → Water used to be drawn from this well by the villagers.

Why the Other Options are Incorrect?

Option 2: The cows were used to be milked by my grandfather every morning. — Incorrect. “Were” cannot come before “used to” when it shows a past habit. “Used to be + V3” is enough.

Option 3: The cows used to milk by my grandfather every morning. — Incorrect. The word “be” is missing. “The cows used to milk” makes the cows do the milking.

Option 4: The cows are used to being milked by my grandfather every morning. — Incorrect. “Are used to being milked” means the cows are now accustomed to it. That is a different meaning, not a past habit.