I will assume that this question asks for the length of the wall on the new blueprint. It is not worded nicely. The actual length of the wall is 12 cm * (2 ft / 1 cm) = 24 ft On the new blueprint of 1 cm = 3 ft, the length on blueprint becomes 24 ft * (1 cm / 3 ft) = 8 cm The length of the wall on the new blueprint is 8 cm.
I would have to say the best answer would be Observational study because it cannot be experimental or nonrandomized studies because those two type of studies require to split a population into a group A or group B. It definitely cannot be a sample survey because it is not a survey. An observational study has lots of variables and tells you the result of the study.