kg and all the given units are units used to measure mass.
kg is the standard unit to measure mass
lets find the value of 0.00684 kg in all the given units
a. dkg
1 kg is equivalent to 100 decagram (dkg)
therefore 0.00684 kg is = 100 dkg/kg x 0.00684 kg = 0.684 dkg
this is not the given value therefore this choice is incorrect
b. 1 kg is equivalent to 1000 g
therefore 0.00684 kg = 1000 g/kg x 0.00684 kg = 6.84 g
this is the given value so this is correct
c. 1 kg is equivalent to 100 000 centigrams (cg)
therefore 0.00684 kg = 100 000 cg/kg x 0.00684 kg = 684 cg
this is the given value so this is also correct
d) 1 kg is equivalent to 1 000 000 mg
therefore 0.00684 kg = 1 000 000 mg/kg x 0.00684 kg = 6 840 mg
this too is correct
the incorrect answer is A. 0.0684 dkg