The correct option is D −−→AC=−−→AB+−−→BC=−−→AD+−−→DC
In a parallelogram −−→AB=−−→DC=→a (say)
and −−→BC=−−→AD=→b
According to the commutative law of vector addition
→a+→b=→b+→a
Hence,
−−→AB+−−→BC=−−→AD+−−→DC
and according to vector addition
−−→AB+−−→BC=−−→AC