Select the correct passive form of the given sentence.

Picasso painted Guernica in 1937.

Guernica was painted by Picasso in 1937.

The correct passive form of the given sentence is Guernica was painted by Picasso in 1937.

The active voice of the sentence is Picasso painted Guernica in 1937. 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 "Guernica" and the main verb is "paint." The past participle of "paint" is "painted." So, the passive form of the sentence is Guernica was painted by Picasso in 1937.

The other options are incorrect because:

• Guernica was painting by Picasso in 1937. is in the present progressive tense, which is used to describe an action that is happening now but is not finished. The painting of Guernica was finished in 1937.
• Guernica was been painted by Picasso in 1937. is grammatically incorrect. The adverb "been" is unnecessary in the passive voice.
• Guernica was paint by Picasso in 1937. is grammatically incorrect. The verb "paint" is missing the past participle ending.