Which of the following is heaviest?

(A) 50gm of Iron

(B)  5 moles of nitrogen= 5x28=140grams (since1 mole of nitrogen = 28grams)

(C) $0.1$ gram atom of silver=$0.1\times \mathrm{At}.\mathrm{wt}. \mathrm{of} \mathrm{Ag}=0.1\times 108=10.8\mathrm{gm}$

(D) 6x ${10}^{23}$ atoms of Carbon = 1 mole =12 grams.

Hence the correct option is (B).