The correct option is D) would.
"Would" is used to describe habitual or repeated actions in the past, which fits a memoir recounting past habits.
In past habit contexts (especially in memoirs/autobiographies), "would" is the correct modal. "Used to" or "would" both express past habits — "could" implies ability, "will/shall" refer to future. Examiners expect students to identify "would" for repeated past actions.