Basmati rice costs $(a-b) per kilogram in standard 7-

1kg of Basmati rice => $( a - b) for standard packages
One standard package = 7kg => $7( a - b)
1kg of basmati rice => $ (a) for non-standard packages
a> b > o => a > o and b > o and a > b
Both a and b are positive integers
Note: standard packages are heavier/greater than non-standard packages
For lowest possible price of exactly 24kg of basmati rice => $$\frac{24kg}{i\ s\tan dard\ \ package}$$
where 1 standard package - 7kg
24kg / 7 kg = 3 remainder 3
Hence, in exactly 24kg, there will be 3 standard package and 3 non-standard package
If the price of 1 standard package = $7 (a - b )
3 standard package = $7 * 3 ( a - b ) = $21 ( a - b )
For non-standard packages;
1 non-standard package = 1kg = $a
3 non-standard package = 3kg = $3a
Total price = price of standard package + price of non-standard package
= [$21 ( a - b ) ] + $3
= $21a - $21b + $3a
= $24a - $21b
Answer= option C