**Core Concept**: Word formation (Vocabulary). **Analysis**: The noun form of 'resign' is 'resignation' (A). - **B. Resigning** is a gerund. - **C. Resigned** is a past participle. - **D. Resigns** is third-person singular verb. **Key Point**: '-ation' suffix commonly forms nouns from verbs. **Common Mistake**: Confusing gerunds with true noun forms.