The correct option is
D Had been planning
NOTE: As the action of 'planning' ended when 'my little sister told me', we will use the past perfect continuous tense, to explain that the action was continuing in the past before another action took place.
Option A - 'Had been planned' is used to represent the sentence in the past perfect tense. Thus option A is incorrect.
Option B - 'Had being planning' is grammatically incorrect, as, for past perfect continuous tense, we use 'been' and not 'being'. Hence, option B is incorrect.
Option C - 'Has been planning' is used to represent a sentence in the present perfect continuous tense, but 'has' is used as singular subject only. As 'my parents' is the plural subject, we can't use 'has' here. Also, we need the sentence to be in the past perfect continuous tense and not in the present perfect continuous tense. Hence, option C is incorrect.
Option D - 'Had been planning' is used to represent a sentence in the past perfect continuous tense. Hence, option D is the correct answer.
'My parents had been planning a surprise party for me, but my little sister told me!'