D
[解析] 词汇辨识题。maintain意为“坚持认为;维持”,例如:He maintained that he was innocent. =He maintained his innocence. 再如:We should maintain friendly relations with neighboring countries. 根据上下文,前一句讲有人认为儿童失忆症是因为人脑中的海马体不成熟,此句讲别的理论的看法,故选项D maintain正确。选项A decline意为“婉拒;衰落”,例如:I offered him a lift to his house, but he declined with thanks. 再如:The party's popularity has declined in the opinion polls. 选项B assess意为“评估”,例如:They assessed the cost of the flood damage at £50,000. 再如:Examinations are not the only means of assessing someone's ability.选项C estimate意为“估计,评价”,例如:It was difficult to estimate how many trees had been damaged.