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

☀ A new pill to stimulate the appetite was discovered.

Explanation for other options:

☀ A new pill to stimulating the appetite were discovered. This option is not in the correct passive voice. It uses "to stimulating" instead of the correct passive construction "to stimulate."

☀ A new pill to stimulating the appetite had been discovered. This option uses "had been discovered," which is in the past perfect tense and does not match the simple past tense used in the original sentence. The correct form should use "was discovered" to match the simple past tense used in the original sentence.

☀ A new pill to stimulating the appetite have been discovered. This option uses "have been discovered," which is in the present perfect tense and does not match the simple past tense used in the original sentence. The correct form should use "was discovered" to match the simple past tense used in the original sentence.