Housing has a 15% weighting factor and has experienced 15% inflation.
0.15 * 15% = 2.25% - this is how much housing has contributed to overall inflation.
If you calculate the same figure for the other 3 options, you'll get a) = 2%, b) = 2%, and c) = 1.5%.
This means that housing has had the greatest influence on inflation and so that is the correct answer.