"Ending on any date other than the said 1 *[30th day of September], then at the option of the assessee the year ending on the day which his accounts have so been made up: - [Provided that where an assessee has been assessed in respect of a particular source of income, profits and gains, he shall not in respect of that source exercise this option so as to vary the meaning of the expression “ previous year ” as then applicable to him except with the consent of the Income-tax Officer and upon such conditions as the Income-tax Officer may think fit : or] (6) in the case of any person, business or company, or class o[ person, business or company, such period as may be determined by the Financial Commissioner or by such authority as he may authorize in this behalf ; 3 [or (c) where a business or profession has been newly set up in the financial year preceding the year for which assessment is to be made, the period from the date of the setting up of the business..."