The word, "I've" is the shortened form of "I have". It suggests that the phrase, "I've ever ______" is in present perfect tense. The format: Subject+ have + verb in past participle form is used in present perfect tense. So, we need to choose the verb which is in its past participle form.
Option A: The word, "ate" is the simple past tense of the verb "eat". The past participle form is "eaten". Hence, this option is incorrect.
Option B: The word, "had" is the past participle form of the verb, "have" , which means possess/ own. The phrase, "I've ever had" means the same as "I have ever eaten". Hence, this option is correct.
Option C: The word, "eat" is the simple present tense of the verb "eat". The past participle form is "eaten". Hence, this option is incorrect.
Option D: The word, "saw" is the simple past tense of the verb "see". The past participle form is "seen". Hence, this option is incorrect.