Option D: Oranges
To determine the cause of sickness, we need to identify the common fruit consumed by both B and C, as they are the ones who fell sick.
     ● Ate by A: Grapes and Pineapple  
     ● Ate by B: Grapes and Oranges  
     ● Ate by C: Oranges, Pineapple, and Apple  
     ● Ate by D: Grapes, Apple, and Pineapple  
     Analysis:
     ● Grapes: Consumed by A, B, and D. Since A and D did not fall sick, grapes are unlikely to be the cause.  
     ● Pineapple: Consumed by A, C, and D. Since A and D did not fall sick, pineapple is unlikely to be the cause.  
     ● Apple: Consumed by C and D. Since D did not fall sick, apple is unlikely to be the cause.  
     ● Oranges: Consumed by B and C. Both B and C fell sick, making oranges the common factor and likely cause of sickness.  
     Therefore, the cause of sickness is most likely oranges.