The correct Active/Passive form of the given sentence is:

☀ A nuanced position on Ukraine was adopted by G20 declaration.

Explanation for other options:

☀ A nuanced position was adopted at G20 declaration on Ukraine. This option is in the passive voice, but the word order is not correct. The correct form should be "A nuanced position on Ukraine was adopted at the G20 declaration."

☀ A nuanced position has been adopted on Ukraine at G20 declaration. This option is in the passive voice, but it changes the verb tense to present perfect ("has been adopted"). The original sentence is in the simple past tense, so the correct form should be "A nuanced position on Ukraine was adopted by the G20 declaration."

☀ A nuanced position is adopted on Ukraine by G20 declaration. This option is in the passive voice, but the verb tense is not correct. The original sentence is in the simple past tense, so the correct form should be "A nuanced position on Ukraine was adopted by the G20 declaration."