Iron nails become ______ after being dipped in copper sulphate solution.

brownish

The correct answer is option 4. brownish.

Iron nails become brownish after being dipped in copper sulfate solution.

When iron nails are dipped in a copper sulfate (CuSO₄) solution, a displacement reaction takes place. The iron (Fe) from the nails displaces copper (Cu) from the copper sulfate solution, forming iron sulfate (FeSO₄) and depositing copper on the iron surface. The copper deposited on the iron gives it a brownish appearance. Over time, the iron may also undergo corrosion, contributing to the overall color change.

So, the correct answer is option 4. Brownish.