1. Interpret 12 ppm of iron in wheat

For solids, $12\,\text{ppm}$ means:

$12\text{ mg of Fe per kg of wheat}$

Given wheat mass $=150\,\text{kg}$, required iron mass is:

$12\times 150=1800\text{ mg}$

So,

$m_{\text{Fe}}=1800\text{ mg}=1.8\text{ g}$

2. Find molar mass of $\mathrm{FeSO_4\cdot 7H_2O}$

$M(\mathrm{FeSO_4})=56+32+4\times 16=56+32+64=152$

$M(7\mathrm{H_2O})=7\times(2+16)=7\times 18=126$

Therefore,

$M(\mathrm{FeSO_4\cdot 7H_2O})=152+126=278\,\text{g mol}^{-1}$

3. Mass fraction of iron in $\mathrm{FeSO_4\cdot 7H_2O}$

One mole of $\mathrm{FeSO_4\cdot 7H_2O}$ contains $56\,\text{g}$ of Fe in $278\,\text{g}$ of compound.

So if $x$ g of $\mathrm{FeSO_4\cdot 7H_2O}$ is needed to supply $1.8$ g Fe:

$\frac{56}{278}x=1.8$

$x=1.8\times \frac{278}{56}$

$x=8.9357\,\text{g}$

4. Nearest integer

$x\approx 9\,\text{g}$

Final Answer

$\boxed{9}$