Marilyn French, a writer and feminist activist whose debut novel, “The Women’s Room,” propelled her into a leading role in the modern feminist movement, died on Saturday in Manhattan. She was 79 and lived in Manhattan. The cause was heart failure, said her son, Robert. With steely views about the treatment of woman and a gift for expressing them on the printed page, Ms. French transformed herself from an academic who quietly bristled at the expectations of married women in the post-World War II era to a leading, if controversial, opinionmaker on gender issues who decried the patriarchal society she...