Answer:
1.
Definite grammatical article that implies necessarily that an entity it articulates is presupposed; something already mentioned, or completely specified later in that same sentence, or assumed already completely specified.
2.
Used before a noun modified by a restrictive relative clause, indicating that the noun refers to a single referent defined by the relative clause.
3.
Used before an object considered to be unique, or of which there is only one at a time.
