Answer: D

Reasoning:  
• COPD is defined by persistent airflow limitation, so forced expiratory indices (FEV1) and maximal expiratory flow rates (MEFR) are characteristically reduced, and air-trapping leads to an increased residual volume (RV).  
• The diffusing capacity for carbon monoxide (DLCO) falls mainly when emphysema destroys alveolar-capillary surface, but it is often normal in chronic bronchitis; therefore a low diffusion capacity is not a uniform or required feature of COPD as a whole [Chou, 1999, PMID 10367481; Yoshimi, 2007, PMID 17419385].  
Hence “decreased diffusion capacity” is the exception.