Answer:
By using Modernist themes instead of doubt and mystery ideas.
Step-by-step explanation:
Robert Frost's poetry represent a "transition" toward Modernist literature due to movement from doubt and mystery ideas to Modernist themes which means he uses Modernist themes instead of doubt and mystery ideas. Robert Frost's poetry moved away from ideas of doubt and mystery into Modernist themes. If the structure is not Modernist so it follow traditional themes wholeheartedly so we can conclude that in Robert Frost's poetry transition occurs.