In the replacement reaction of silver nitrate (AgNO3) and potassium sulfate (K2SO4), the reaction can be represented as follows:
\[ 2AgNO3 + K2SO4 \rightarrow Ag2SO4 + 2KNO3 \]
Therefore, the substance produced alongside silver sulfate (Ag2SO4) is 2KNO3 (potassium nitrate).
So, the correct answer is:
2KNO3