Final answer:
The prefix meaning "painful" is (a) Dys, which suggests something that is abnormal, impaired, or difficult. Other options presented, such as Dis, Dia, and De, have different meanings suggesting separation, crossing, or reduction, respectively.
Step-by-step explanation:
The prefix that means "painful" is option (a) Dys. This prefix originates from Greek and is commonly used in medical and psychological terminology to indicate something that is abnormal, impaired, or difficult. For example, dysfunctional means not operating normally or properly, dyspepsia refers to indigestion or pain in the digestive system, and dyslexia describes a learning disorder characterized by difficulty in reading.
In contrast, the prefix (b) Dis typically means apart, away or not, as seen in words like disapprove or disappear. The prefix (c) Dia means across, though, or between, as in diagnosis or diagonal. Lastly, the prefix (d) De often implies removal or reduction, found in words such as dehydrate or decline.