What defines a mixture?
a. a mixture cannot be broken down into simpler substances.
b. a mixture can be broken down into its basic substances through physical means.
c. a mixture is created through a chemical reaction.
d. a mixture can be described by a chemical formula.?

b.a mixture can be broken down into its basic substances through physical means.

Explanation:

The substances in a mixture can be separated using physical methods such as filtration, freezing, and distillation. There is little or no energy change when a mixture forms. Mixtures have variable compositions, while compounds have a fixed, definite formula.

its first and third is your answer i did the qustion the fourth isnt right

answer is: naturally occurring solid, inorganic compound or element is mineral.

minerals usually have crystalline form, that means than crystal constituents (atoms, molecules or ions) are arranged in a highly ordered microscopic structure, forming a crystal lattice that extends in all directions.

for example magnetite is a mineral of iron oxide (fe₂o₃).

calcite is a carbonate mineral and the most stable polymorph of calcium carbonate (caco₃).

dolomite is an   carbonate mineral composed of calcium magnesium carbonate (camg(co₃)₂).