The correct option is E small importance
"Our object, in setting forth this work, is one of no mean moment; and we trust that every Australian, whether this is his native or adopted country, will heartily bid us "God speed!"" This sentence , here, means to say that the objective of writing this article is of no small importance and that the writers believe that every Australian- native or adopted- will wish them luck in achieving their their objective. From the context of the sentence extracted from the passage it is evident that the phrase 'no mean moment' means of no small importance. Thus, 'mean moment' can be rephrased as small importance. E is the correct answer. The other options do not fit the context.