Divide the load by the longest side eg. 400kg / 1.2 = 333.33 so this is in limits as the question gives a maximum load per running metre of 350kg/m.
Then to check the Floor strength limit which is 300kg per square metre: Length X Width so eg: 1.2 X 1.2 = 1.44 400 / 1.44 = 277.77 per square metre also within limits.
If you repeat this for each option then you can discard all the others either for the linear limit or the floor strength. Take the 700 kg crate: 700 / 1.8 (as this is the longest side) = 388.88 can be discarded as over 350. etc....