In the metric system of units: 1 meter=100 centimeters and 1 meter: 1 foot=3.25: 1

As the question is incomplete, the complete question is;
How many meters are there in 21 feet? Note: In the metric system of units: 1 meter = 100 centimeters and 1 meter:1 foot = 3.25:1
The ratio of 1 meter to 1 foot is given that is 3.25:1
meter  :   foot
3.25    :  1
when there is 21 foot, it means multiply both sides of ratio with 21
3.25 x 21 : 1 x 21
68.25 :21
Thus, if there is 21 foot, it means it is equal to 68.25 meters.
And if convert meter to centimeter;
68.25m = 68.25 x 100 cm
= 6825 cm

The answer to your question is the last one