As a congressional aide for a senator, one of Ian's key responsibilities would likely be "Researching and writing the language for an environmental regulation." This task involves gathering information, analyzing existing laws and regulations, and drafting proposed legislative language to address environmental issues, which is a common function of legislative aides.
On the other hand, "Auditing the accounts of a government agency" is more typically the responsibility of financial auditors or comptrollers rather than legislative aides. Therefore, the research and writing task aligns more closely with the duties of a Senate aide.