答案：D

解析：

The highest attendance was at the spring 2011 meeting, which had 72 people attending. Since only members can attend the meetings, these 72 people must have been members of the Maple Street Block Association; thus, there must have been at least 72 members of the Block Association. If the 65, 61 and 70 people who attended the winter, summer and autumn meetings, respectively, were all among the 72 people who attended the spring meeting, then the Block Association could have had as few as 72 members in all. Therefore, based on the bar graph, the least number of members the Maple Street Block Association could have had in 2011 is 72.