$a)$

$10m^2=10\cdot 100cm\cdot 100cm=100000c{m}^2$

$5m^2=5\cdot 100cm\cdot 100cm=50000c{m}^2$

$1,6m^2=1,6\cdot 100cm\cdot 100cm=16000c{m}^2$

$23,5m^2=23,5\cdot 100cm\cdot 100cm=235000c{m}^2$

$4d{m}^2=4\cdot 10cm\cdot 10cm=400c{m}^2$

$15d{m}^2=15\cdot 10cm\cdot 10cm=1500c{m}^2$

$120d{m}^2=120\cdot 10cm\cdot 10cm=12000c{m}^2$

$1,4d{m}^2=1,4\cdot 10cm\cdot 10cm=140c{m}^2$

$140m{m}^2=140\cdot 0,1cm\cdot 0,1cm=1,4c{m}^2$

$1500m{m}^2=1500\cdot 0,1cm\cdot 0,1cm=15c{m}^2$

$7a=700m^2=700\cdot 100cm\cdot 100cm=7000000c{m}^2$