The word that best defines the italicized word (capacious) from the novel Frankenstein is vast. The last option is correct.
Capacious can be defined as having a lot of space inside; roomy.
Vast means of very great extent or quantity; immense.
Frankenstein; or, The Modern Prometheus is a novel written by English author Mary Shelley that follows the story of Victor Frankenstein, a young scientist who creates a hideous creature in an unorthodox scientific experiment.