。
How did they react to the report?
Literal
Report [toward-に対する] their reaction [topic-は] how was-[polite-past-でした] [question-か]?
に対する is the adnominal form of に対して (toward/regarding) — it modifies 反応 (reaction). More formal than just への. The question word どう (how) + でしたか asks about the nature of the reaction.