The correct option is
C She is not only a smart politician, but also attentive to the needs of the people.
The conjunction, 'Not only.. but also' is a correlative conjunction, which combines two grammatically equal items. This means that the words which follow both parts (i.e 'not only' as well as 'but also') should belong to the same parts of speech. Using different parts of the speech after 'not only' and 'but also' makes the sentence imbalanced and grammatically incorrect.
Option A: The phrase, 'not only' is followed by 'a smart politician' - an adjective, which describes the pronoun, 'she'. The phrase 'but also' is followed by 'attentive to the needs of the people' - an adjectival phrase, which describes the pronoun 'she'. Hence, this option is grammatically correct.
Option B: In this sentence, the phrases 'not only' and 'but also' are used together, which is grammatically incorrect.
Option C: When a sentence begins with 'not only, it is followed by an auxiliary verb like 'did'/does'/'is' + subject. In this option, the sentence begins with 'Not Only'. However, it is not immediately followed by an auxiliary verb. Hence, this option is incorrect.
Option D: The phrase, 'not only' is followed by 'is', a verb. However, the phrase 'but also' is not used in the sentence. Hence, the sentence is incomplete and is grammatically incorrect.