The statement, "energy can neither be created nor destroyed, but can change from one form to another," is
the first law of thermodynamics
the second law of thermodynamics
the third law of thermodynamics
scientifically unproven

The first law of Thermodynamics

Explanation:  The law states that energy can never be destroyed, or created.  This is because of a cycle of atoms, atoms are always changing, never getting destroyed.

The first law of thermodynamics

Explanation:

The first law of thermodynamics, also known as Law of Conservation of Energy, states that energy can neither be created nor destroyed; energy can only be transferred or changed from one form to another. For example, Humans can convert the chemical energy in food, like an ice cream cone, into kinetic energy by riding a bicycle. Plants can convert electromagnetic radiation (light energy) from the sun into chemical energy.

not necessarily. higher fitness requires the organism to be reproductive successful. for example, an organism at age 10 with 15 offspring is much more fit than an organism at age 15 with only 5 offspring.

some inherited genetic mutations can be good for the offspring, so the answer is different genetic mutations cause many forms of cancer.