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

☀ They sent for a doctor because Paula had fainted.

Explanation for other options:

☀ They sending for a doctor because Paula had fainted. This option is not grammatically correct. The correct form should be "They sent for a doctor," not "They sending for a doctor."

☀ They were sending for a doctor because Paula has fainted. This option changes the tense of the verb from past to present, which alters the meaning of the sentence. The correct tense in this context is the past tense "had fainted."

☀ They have been sending for a doctor because Paula had fainted. This option uses the present perfect continuous tense "have been sending," which is not appropriate in this context. The correct form should use the simple past tense "sent" to match the original sentence.