First, we should convert everything into meter
a) 2.5m + 3m + 1.1m
b) 3m + .25m + 2.4m + 1m
c) 3m + .3m + 2.1m + 1.1m
d) 3.2m + 1.1m + 1.2m
And it looks like answer D will give you the correct answer. If you want to know how I got the units, here's how:
1m = 10dm 1m = 10dm = 100 cm = 1000mm
1dm = 10cm
1cm = 10mm