#### Answer

a. $100~m^2$

#### Work Step by Step

If a square garden had sides measuring 10 m, the area would be $100~m^2$. Since these dimensions seem reasonable for the size of a flower garden, this is a good estimate for the area of a flower garden in a person's yard.
If a rectangular garden had sides measuring 0.4 hm and 1 hm, the area would be $0.4~hm^2 = 0.4~ha$. Since these dimensions are similar in size to a football field, this seems too large for the area of a flower garden in a person's yard.
If a square garden had sides measuring 0.1 km, the area would be $0.01~km^2$. Since this garden has sides measuring 100 meters, this seems too large for the area of a flower garden in a person's yard.
The best estimate for the area of a flower garden in a person's yard is $100~m^2$.