The correct active/passive form of the given sentence is Reena said that she would have to work hard to finish that project within the stipulated time.

The sentence is in the simple future tense, which is used to describe an action that will happen in the future. The active voice of the sentence is formed by putting the subject of the sentence before the verb and removing the word "by." In this case, the subject of the sentence is "Reena" and the verb is "say." So, the active form of the sentence is "Reena said that she would have to work hard to finish that project within the stipulated time."

The other options are incorrect because:

• Reena said that he would have to work hard to finish that project within the stipulated time.” This is incorrect because the word "he" refers to the speaker, but the speaker is Reena.
• Reena said that she would had to work hard to finish this project within the stipulated time.” This is incorrect because the word "had" is in the past perfect tense, which is used to describe an action that happened in the past and is still relevant to the present. The sentence is about an action that will happen in the future.
• Reena said that she would have to work hard to finish this project within the stipulated time.” This is incorrect because the word "this" refers to the present project, but the project is supposed to be finished within a stipulated time.
• Reena said that he would had to work hard to finish that project within the stipulated time.” This is incorrect because the word "he" refers to the speaker, but the speaker is Reena.