The narrative voice of the short story "Through the Tunnel" by Doris Lessing is third person. More specifically, the story is written in the third person omniscient voice. The third person is clear through the use of pronouns such as "he," "she," and "they."
He was an only child, eleven years old. She was a widow. She was determined to be neither possessive nor lacking in devotion. She went worrying off to her beach.
If the story was written in the first person voice, then narrator of the story would be Jerry or his mother. The above text would read something like this: "I was an only child. My mom was a widow and determined to not be possessive of me."
The omniscience part of the narration is because the narrator knows what each character is thinking about. The narrator explains Jerry's thoughts in equal detail to how mother's thoughts are explained. Because of this voice, the reader knows all of each character's motivations.