Question

An element is defined by –
(A) Chemical bonds
(B) Electrons
(C) Neutrons
(D) Protons
(E) Ions

Answer 1

An element is a substance whose atoms all have the same number of protons: another way to say this is that all the atoms of a specific element have the same atomic number.

Complete answer:
Elements are the simplest substances in chemistry and can therefore not be broken down using chemical reactions. It is only possible to convert elements into other elements using nuclear methods.
If an element, as part of a dissolved molecule, is in a bonded relationship with another element or compound, is in solution, or is under electrical or magnetic stress or influence, an electron can be lost or gained. That makes it an ion. So, option (A.) is incorrect.
In atoms of the same element, the number of electrons can also be different, thus producing ions (charged atoms). For instance, in its neutral state, iron, Fe may exist in $+2$ and $+3$ ionic states. So, option (B.) is incorrect.
To produce isotopes, which are atoms of the same element that have different neutron numbers, the number of neutrons can vary. It is also defined as variants of a specific element, where the number of protons in these variants will be the same but the number of neutrons in the atom will differ. So, option (C.) is incorrect.
An element's neutral atoms contain an equal number of protons and electrons. The number of protons determines the atomic number (Z) of an element and differentiates one element from another. For example, in this case, the atomic number (Z) of carbon is $6$ because it has $6$ protons. So, option (D.) is correct.
Ions are atoms with charges that may be positive or negative based on the number of electrons it gained or released. For the definition of elements, ions cannot be considered. So, option (E.) is incorrect.
Hence, the correct option is (D.)

Note:
The best approach is that under normal conditions other than radioactive decay or under abnormal circumstances of induced nuclear (nucleus being where the protons are) forces of fission (breaking) or fusion (combining), the number of protons in the atom cannot change.