Step 1: Understanding the Concept:
This question tests knowledge of different forms of a word (parts of speech) and which form fits grammatically into the sentence. The blank needs a word to describe the noun "act."
Step 2: Detailed Explanation:
1. Analyze the Sentence Structure: The structure is "a [adjective] act". The article "a" and the noun "act" require an adjective to modify or describe the noun.
2. Examine the Word Forms:
- (A) pardon: This is a noun (an act of forgiveness) or a verb (to forgive). It cannot be used as an adjective here. ("a pardon act" is incorrect).
- (B) pardoning: This is a present participle. While participles can be used as adjectives (e.g., "a charming person"), "a pardoning act" would mean an act that pardons someone, which doesn't fit the context.
- (C) pardoned: This is a past participle. "A pardoned act" would mean an act that has been forgiven, which is semantically awkward.
- (D) pardonable: This is an adjective ending in "-able," which means "capable of being pardoned" or "excusable." This fits perfectly to describe the nature of the "act." "It was not a pardonable act" means the act was not excusable.
Step 3: Final Answer:
The correct adjective to modify "act" is (D) "pardonable".