Final answer:
True(a), the brain of Stegosaurus was indeed very small, roughly the size of a walnut, which is common in many large dinosaur species.
Step-by-step explanation:
The statement that the brain of Stegosaurus was no larger than a walnut is true(a). Despite their large body size, Stegosauruses had relatively small brains. This is common in a number of dinosaur species, where there is not always a direct correlation between body size and brain size. For comparison, the human brain is significantly larger relative to body size; for example, Homo naledi, an ancient human relative, had a brain size no larger than an average orange, which is larger than the brain size of a Stegosaurus despite Homo naledi being a much smaller creature.