12 mm is how much percentage of 18 cm ?

The correct answer is (c).

To calculate the percentage, we divide the part by the whole and multiply by 100. In this case, the part is 12 mm and the whole is 18 cm. So, we have:

$12 \text{ mm} \div 18 \text{ cm} \times 100\% = 6.67\%$

Therefore, 12 mm is 6.67% of 18 cm.

Option (a) is incorrect because 6.7% is not equal to 6.67%.

Option (b) is incorrect because 6.6% is not equal to 6.67%.

Option (d) is incorrect because 7.0% is not equal to 6.67%.