We are converting a sentence from direct speech to reported (indirect) speech.
The reporting verb is said, which is in the past tense. Therefore, we need to make changes to pronouns, modals, and words indicating proximity.
1. Pronoun Change: The pronoun 'I' refers to the speaker ('she'), so it changes to 'she'.
2. Modal Verb Change: The modal verb 'can' changes to its past form, 'could'.
3. Proximity Word Change: The word 'this' changes to 'that'.
Applying these changes to the original sentence: 'I can finish this project by Monday.'
The reported speech becomes: She said that she could finish that project by Monday. (The word 'that' is optional).
This exactly matches option (A).