Explanation:

When using indirect or reported speech, the form changes.Usually indirect speech is introduced by the verb said,If the main verb is already in a past tense, then the tense changes to another past tense; it can almost be seen as moving even further into the past.Now in the sentence The boss said to his employee, "I will be happy if you finish this project by evening." The verb said is in direct form so we will have to change it in indirect form of speech so the word 'told' is used. The whole sentence is in the past tense so there can be no use of future or present tense. Hence option 3 is the most suitable answer for the given sentence.