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

☀ No one has climbed this mountain before.

 Explanation for other options:

☀ No one has been climbed this mountain before. This option is not grammatically correct. The correct form should be "No one has climbed this mountain before," not "No one has been climbed this mountain before."

☀ No one had been climbed this mountain before. This option is not grammatically correct. The verb tense "had been climbed" is not appropriate in this context. The correct form should use the present perfect tense, as in "has climbed."

☀ No one had been climbing this mountain before. This option uses the present perfect continuous tense "had been climbing," which is not appropriate in this context. The correct form should use the simple present perfect tense, as in "has climbed."