“They wanted X which were placed high” is a tense problem, not a “which” problem. It swaps from “future in the past” (they wanted X) to past “which were placed” (which were the whole world put).

What would be the correct option if “they wanted X placed high”, “they wanted X to be placed high”, “they wanted X, which were to be placed high”, or “they wanted X, which would be placed high”.

Edit: Similarly, A) is wrong because “placing” is present tense and doesn’t match “they wanted”, which is past tense, and requires the “to place” tense to either be past or “future in the past”.