In which literary form did Gertrude Stein primarily write?

Gertrude Stein is best known for her contributions to poetry and experimental novels. Throughout her career, she pushed the boundaries of traditional narrative structures, often employing innovative language and form that challenged conventional storytelling. This experimental approach is evident in works such as "Three Lives," where she utilized repetitive language and a fragmented style that broke away from realistic representation.

Her poetry, marked by its emphasis on rhythm and sound over traditional narrative, further exemplifies her focus on the abstract and the avant-garde. Stein's unique contributions to literature led her to be an influential figure in the modernist movement, making her works distinct in their style and form.

While historical fiction, biographies, and simple narratives aimed at children are all legitimate literary forms, they do not align with Stein’s primary body of work. Her focus on poetry and experimental novels defines her literary legacy and influence on future generations of writers. This distinction is what makes the answer centered on her engagement with poetry and avant-garde narrative styles the most accurate choice.