💡 Detailed Explanation
The correct answer is "といっても". The expression "まさに～といっても過言ではない" is an N1-level fixed phrase meaning "it is no exaggeration to say that it is truly～." It asserts that the preceding statement is so true that calling it an overstatement would be wrong, emphasizing the truthfulness or significance of the statement.
Let's examine the other options:
- "とはいえ" (although; even so): While grammatically possible after a plain form, it implies a concession or contrast. "不可欠だったとはいえ過言ではない" would mean "Although it was indispensable, it's not an overstatement," which is semantically awkward and does not form the intended fixed expression. The nuance of "although" doesn't align with the idea of 'it's no overstatement to say'.
- "とあれば" (if it's the case that; given that): This conditional structure does not fit the context of the fixed expression "～過言ではない." "不可欠だったとあれば過言ではない" would imply 'if it was indispensable, then it's not an overstatement,' which is illogical as the 'overstatement' refers to the act of saying, not the condition itself.
- "とばかりに" (as if to say; as if): This expression indicates that something is done as if to convey a certain message or feeling. "不可欠だったとばかりに過言ではない" is grammatically and semantically incoherent with "過言ではない." It doesn't connect logically to the 'not an overstatement' part.