Option a- ‘Risen’ refers to arise or reach the next position. Example- the recent air balloon rose within the air. ‘Risen’ is different in aiming to lessened. Hence, it's an incorrect option.
Option b- ‘Diminished’ refers to the method of reducing or decreasing. Example- The supreme court diminished the powers of the local MPs. ‘Diminished’ is analogous in aiming to lessened. Hence, it's the right option.
Option c- ‘Extended’ means to enlarge or expand something in size or shape. Example- We are planning on extending our garden. ‘Extended’ is that the opposite in intending to lessened. Hence, it's an incorrect option.
Option d- ‘Increased’ refers to enlarging or expanding something. Example- the govt has increased the quantity of taxes. ‘Increased’ is that the opposite in aspiring to lessened. Hence, it's an incorrect option.

The correct answer is Option ‘b’.