Objects A and B repel each other but attract C. C repels D. If D is positively charged, then the charge on B is:

A charge is a fundamental property of matter by which an electric field is experienced. There are two types of charges, one is a positive charge, and another is a negative charge. The same charges repel each other, and opposite charges attract each other. 

Here, in this case, C is repelled by D, and D is positively charged, so C will also be positively charged. C is attracted by A and B, and then A and B will be negatively charged because opposite charges attract each other. 

Therefore, the charge on B is negative. Hence, Objects A and B repel but attract C, and C repel D. If D is positively charged, then, the charge on B is negative.