The correct answer is: Only assumption II is implicit.
Assumption I is not implicit. The statement only says that the X brand has the largest sale, not that the sale of all the radio sets manufactured in India is known. For example, it is possible that the X brand has a large sale because it is the only brand of radio set manufactured in India.
Assumption II is implicit. The statement says that the X brand has the largest sale, which means that the sale of all the other brands of radio sets manufactured in India is smaller. This implies that the manufacturing of no other radio set in India is as large as ‘X’ brand radio.