Which technique isn't useful for single source circuits? loop equations,superposition, thevenin's theorem, or norton's theorem?
Superposition. Supersposition theorem is used to analyze the circuits using only one source of power at a time. In a single-source circuit there is only one source of power to begin with, so there is no point in applying the superposition theorem.
Loop equations, Thevenin theorem and Norton's theorem are all used to simplify the circuits containing multiple resistors, so they can be useful no matter how many sources of power a circuit in question has.