Select the correct passive form of the given sentence.

The manager lost the letter.

The letter was lost by the manager.

The correct passive form of the given sentence is The letter was lost by the manager.

The active voice of the sentence is The manager lost the letter. This sentence is in the simple past tense, which is used to describe an action that happened in the past. The passive voice of the sentence is formed by putting the subject of the active sentence after the verb "to be" and the past participle of the main verb. In this case, the subject of the active sentence is "letter" and the main verb is "lose." The past participle of "lose" is "lost." So, the passive form of the sentence is The letter was lost by the manager.

The other options are incorrect because:

• The letter was loss by the manager. is grammatically incorrect. The word "loss" is a noun, not a verb.
• The letter was loose by the manager. is grammatically incorrect. The word "loose" means "not tight." It does not mean "lost."
• The letter was lose by the manager. is grammatically incorrect. The verb "lose" is missing the past participle ending.