It has been given that the interior angles in a polygon are in an arithmetic progression. We know that the sum of all exterior angles of a polygon is 360°. Exterior angle = 180° - interior angle Since we are subtracting the interior angles from a constant, the exterior angles will also be in an AP The starting term of the AP formed by the exterior angles will be 180°-120° = 60° and the common difference will be - 5°. Let the number of sides in the polygon be 'n'. => The number of terms in the series will also be 'n'. We know that the sum of an AP is equal to 0.5*n*(2a+(n−1)d), where 'a' is the starting term and 'd' is the commondifference. 0.5*n*(2*60°+(n−1)*(−5°))=360° 120−5n2+5n=720 5n2−125n+720=0 n2−25n+144=0 (n−9)(n−16)=0 Therefore, can be 9 or 16. If the number of sides is 16, then the largest external angle will be 60−15*5=−15°. Therefore, we can eliminate this case. The number of sides in the polygon must be 9. Therefore, option C is the right answer.