When evaluating drugs to be used to treat a particular disease, which of these values is important in terms of determining the relative safety of the drug? a. xd-50 and ld-50 b. ld-50 and td-50 c. ed-50 and ld-50 d. td-50 and ed-50

The answer to this question would be D. TD-50 and ED-50
In drug therapeutic index, ED mean effective doses, TD mean toxic doses and LD mean lethal doses. Drug safety will be determined by lethal doses / effective doses. But, in a drug that used for human determining the "lethal doses" would be impossible to pass ethics since he research would kill someone. So, the drug used for human calculation would use the toxic doses.

By knowing the ED-50 and TD-50, you can find the range where the drug can work but won't cause a toxic effect. The larger the range, the safer it will be. A drug with narrow safety like lithium will need a constant monitor of drug concentration.