Detailed Explanation:

Paramagnetic substances are weakly attracted by magnetic fields and have small positive susceptibility (χ > 0). Among the options, platinum is a classic paramagnetic material with unpaired electrons that align weakly with external magnetic fields.

Iron and nickel are ferromagnetic (strongly magnetic, χ >> 1000), while mercury is diamagnetic (weakly repelled, χ < 0). Other paramagnetic examples include aluminium and manganese.

Therefore, only platinum exhibits paramagnetic behavior, making Option 3 the correct answer.