The Gaussian plume model earns its simplicity by assuming away exactly the kinds of complexity that would otherwise require a full numerical simulation, and checking each option against that simplifying spirit quickly separates the valid assumptions from the invalid ones. The model treats the pollutant as conservative, meaning once it leaves the stack, the total mass in the plume cross-section stays the same as it spreads out, with no chemical breakdown, radioactive decay, or ground deposition removing mass along the way, which matches option A. It also assumes steady-state meteorology: the wind blows from one constant direction at one constant speed, and the atmospheric stability class does not change during the time it takes the plume to travel to the receptor, which lets the whole problem be solved with one straight-line plume centerline instead of a constantly bending one, matching option B. In contrast, the basic model is explicitly built for flat, open, unobstructed terrain; the moment hills, valleys, or buildings are introduced, the airflow develops turbulence, channeling and recirculation that violate the model's smooth Gaussian spreading assumption, so option C describes a scenario the model assumes does NOT happen, not one it assumes does. Likewise, a source with an emission rate that changes significantly during the averaging period would break the steady-state assumption altogether, contradicting option B rather than supporting a fourth, separate valid assumption, so option D is also false. This leaves only the constant-wind, conservative-pollutant pair as genuinely valid assumptions.
\[\boxed{\text{Correct options: A, B}}\]