Final answer:
The strength of a drug required to produce its desired effect is known as its potency. Tolerance refers to the need for increasing doses of a drug to achieve the same effect. The Effective dose - 50% is the dose that elicits a significant response in 50% of subjects.
Step-by-step explanation:
The term that describes "the strength of a drug" required to produce the desired effect is potency. Potency of a drug can be influenced by various factors, including the chemical nature of the drug and the response of the individual to the drug. On the other hand, tolerance occurs when a drug user requires more and more of a given drug to experience the same effects previously achieved with lower doses.
The concept of Effective dose - 50% refers to the dose that results in a significant response in 50% of subjects (option D). This indicates how much of the drug is required to achieve the desired effect in half the population studied, and is a standard measure in pharmacology to assess drug potency.
Psychological dependence refers to drug craving, where the user has an emotional need for the drug, often for the purpose of relieving psychological distress. Meanwhile, physical dependence involves the body adapting to the drug, leading to withdrawal symptoms upon cessation.