The correct answer is D. has become/is.
The practice of working women has become so widespread that the working mother is now not an unusual factor in the world. This is because women have become more educated and have more opportunities to work outside the home. In addition, the cost of living has increased, making it necessary for both parents to work in order to support their families.
Option A, becomes/was, is incorrect because the practice of working women has already become widespread. Option B, became/is, is incorrect because the practice of working women has not just recently become widespread. Option C, had become/is, is incorrect because the practice of working women has not always been widespread. Option E, became/was, is incorrect because the practice of working women is still widespread today.