Question

which of these is a pure substance?
a iron (fe)
b salt water
c salad
d (partially visible, likely air or similar)

Explanation:

A pure substance is made of only one type of particle (element or compound). Iron (Fe) is an element, so it's a pure substance. Salt water is a mixture of salt and water, salad is a mixture of various ingredients, and air is a mixture of gases.

Answer:

A. Iron (Fe)