She hard enough.

Grammar rule:

To form the present continuous tense, we use the auxiliary verb 'to be' (am, is, are) followed by the present participle of the main verb (verb + -ing). The pronoun 'She' is singular and third person, so we use 'is'.

Solution:

She is writing hard enough.

Note: While grammatically correct, the sentence 'She is writing hard enough' might not be the most common or contextually appropriate. However, based solely on the provided options and sentence structure, 'is writing' is the most fitting choice to complete the sentence grammatically.