In the following question, out of the given four alternatives, select the one which is opposite in meaning to the given word. Efficacious – choose the option that is most nearly opposite in meaning.

Introduction / Context:
This question tests knowledge of a slightly formal English word, "efficacious", and the ability to choose its antonym. Many exam words are drawn from academic or formal registers, especially from fields like medicine, law, or philosophy. "Efficacious" is often used to describe treatments, remedies, or methods that produce the desired result effectively.

Given Data / Assumptions:

• The given word is "Efficacious".
• Options: Powerful, Useless, Successful, Good.
• The task is to choose the word that is opposite in meaning.
• We assume the common dictionary meaning of efficacious: effective, capable of producing a desired result.

Concept / Approach:
"Efficacious" describes something that works well and achieves its intended effect. Therefore, its opposite must describe something that fails to work or has no useful effect at all. Among the options, "Useless" clearly expresses the absence of effectiveness. The other options are either positive in a similar way (powerful, successful, good) or at least not opposite.

Step-by-Step Solution:
Step 1: Recall that "efficacious" means effective in producing a desired result, especially in the context of remedies or methods. Step 2: Option B, "Useless", means having no use or benefit and failing to achieve any result. This is a natural opposite of "effective". Step 3: Option A, "Powerful", describes something with great strength or influence, which often suggests high effectiveness, not the opposite. Step 4: Option C, here written as "Successful", refers to achieving a desired aim or result, again very close to "efficacious". Step 5: Option D, "Good", is broad and positive. While it is not a precise synonym, it certainly is not opposite to "efficacious". Step 6: Hence, the only true antonym in the list is "Useless".

Verification / Alternative check:
Consider the phrase "an efficacious medicine" and try replacing the word. "A powerful medicine" and "a successful medicine" still suggest that the medicine works well. "A good medicine" also implies positive effect. However, "a useless medicine" clearly means it does not work at all. Because antonym questions require a clear contrast, the pair "efficacious–useless" fits perfectly.

Why Other Options Are Wrong:
Option A: "Powerful" often suggests strong effect, which aligns more with efficacy rather than opposing it. Option C: "Successful" means achieving the intended result, so it is very similar in meaning to efficacious, not opposite. Option D: "Good" is a general positive adjective and does not capture the idea of complete failure that a true antonym would require.

Common Pitfalls:
Because several options sound generally positive, some students may confuse the direction of the question and pick a near synonym instead of an antonym. They might choose "powerful" or "successful" if they think the exam is asking for a similar word. It is therefore essential always to notice the key word "opposite" or "antonym" before evaluating the options. Another pitfall is not knowing the exact meaning of "efficacious" and guessing based only on the appearance of the word, which can lead to random errors.

Final Answer:
The word that is opposite in meaning to "Efficacious" is Useless.