Tonnage tax option remains enforceable but lapses if the company ceases qualifying, defaults, is excluded, or opts out. An option for the tonnage tax regime, once approved and exercised, remains effective for a fixed ten-year period from the date of exercise and is applied from the assessment year relevant to the previous year of exercise. The option ceases for the assessment year relevant to the previous year in which the company stops being qualifying, defaults on scheme compliance obligations, is excluded from the scheme, or furnishes a written declaration electing to withdraw so profits are computed under the Act's other provisions.