The correct passive form of the sentence is:

☀ The meaning of the culture was being lost.

So, the correct passive form is: "The meaning of the culture was being lost."

The other options and their explanations:

☀ The culture lost it's meaning.

• This option changes the sentence to the active voice, with "the culture" as the subject performing the action "lost."

☀ The meaning of the culture was lost.

• This option is in the passive voice and is close to the correct answer, but it does not include the continuous nature of the action indicated by "was being lost."

☀ The meaning of the culture will lost.

• This option contains a grammatical error. The correct form should be "will be lost" to form the future passive tense. Additionally, it does not match the tense used in the original sentence, which is in the past tense.