Step 1: Analyze the verb structure. The sentence uses the construction "will be ...", which is the form of the Future Continuous tense.
Step 2: Understand the Future Continuous tense. This tense is formed with "will be + the present participle (the -ing form of the verb)". It is used to describe an action that will be in progress at a certain time in the future.
Step 3: Identify the present participle of 'leave'. The present participle is 'leaving'.
Step 4: Complete the sentence. The correct sentence is: "I will be leaving for Chennai next week."