Answer
Only (c) is true, and rest-- (a), (b),and (d) --are false.
Work Step by Step
When iron rusts, oxygen present in the air chemically combines with the iron to form ferric oxide.
4Fe + $3O_{2}$ $\rightarrow$ $2 Fe_{2}O_{3}$
Molar mass of iron (Fe) = 55.8450 g/mol
And molar mass of $ Fe_{2}O_{3}$ is = 159.69 g/mol.
Thus there is a considerable difference in the mass of iron and iron oxide. Thus, the balance will show the mass of the bar greater than 75 grams.
(a) and (b) are false as they contradict just what we proved above.
(d) is also false, as, if we scrape off the rust, the iron content of the bar (now rust) will aslo be scraped off. Thus the reading will be less than 75 grams.
