In the loop shown, if you traverse clockwise starting from the lower left corner, crossing the source from its positive terminal to negative terminal contributes 
A$+V_s$
B$-V_s$
C$0$
DEither $+V_s$ or $-V_s$, independent of direction
Answer & Solution
Correct answer: B. $-V_s$
Crossing a source from positive to negative terminal is a potential drop, so its contribution is negative. Therefore the term is $-V_s$ for that traversal direction.
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