The most stable carbocation is:
(A) Methyl carbocation
(B) Primary carbocation
(C) Secondary carbocation
(D) Tertiary carbocation

Hint: Carbocation: The carbon containing positive charge is known as carbocation. The stability of carbocation is as follows: tertiary carbocation is more stable than secondary than primary carbocation i.e. $ {3^ \circ } > {2^ \circ } > {1^ \circ } $ .

Complete answer:
First of all let us talk about what is carbocation and how they are formed.
Carbocation: The carbon containing positive charge is known as carbocation. When the carbon of any loses its electron then it forms the carbocation.
Primary carbocation: The carbocation which is attached to only one carbon atom, is known as primary carbocation.
Secondary carbocation: The carbocation which is attached to two carbon atoms, is known as secondary carbocation.
Tertiary carbocation: The carbocation which is attached to three carbon atoms, is known as tertiary carbocation.
Methyl carbocation: The carbocation which has two hydrogen atoms and no carbon atom, is known as methyl carbocation.
The stability of these carbocation can be arranged by $ + I $ effect.
 $ + I $ effect: When a chemical species has a tendency to release or donate electrons then charge is relayed through the chain, this effect is known as positive inductive effect or $ + I $ effect. For example: alkyl groups show positive inductive effects.
Now the greater the alkyl group greater will be the effect of positive inductive effect and hence greater will be the stability of the carbocation.
Now, the positive inductive effect of tertiary is greater than that of secondary which is greater than that of primary. So the stability order is:
Tertiary $ > $ secondary $ > $ primary $ > $ methyl carbocation.
Therefore, option (D) is correct.

Note
 $ - I $ effect: When a chemical species has a tendency to accept electrons then charge is relayed through the chain, this effect is known as negative inductive effect or $ - I $ effect. For example: when halogen is introduced to a chain then they show negative inductive effects.