In this question on vocabulary and usage, the sentence "She was completely nonplussed by the erratic behaviour of her father" is given. Choose the option that best improves the underlined expression, if any improvement is needed.

Introduction / Context:
This sentence improvement item tests your understanding of the word nonplussed and its correct usage. The sentence She was completely nonplussed by the erratic behaviour of her father describes a situation where a person is very surprised and confused by someone's unpredictable actions. Many learners are unsure about the meaning of nonplussed and may think it means unconcerned, but in standard English it means so surprised and puzzled that one is not sure how to react. The question asks whether the phrase completely nonplussed needs improvement, and the correct answer is that it does not.

Given Data / Assumptions:
The key points in the sentence are as follows.

• Subject: She, referring to a daughter or another person.
• Verb: was, linking the subject to a state.
• Underlined phrase: completely nonplussed.
• Cause: the erratic behaviour of her father.
• Intended meaning: she was extremely puzzled and at a loss for words or actions.

Concept / Approach:
Nonplussed is an adjective meaning extremely puzzled, bewildered, or at a loss for what to do or say. It comes from a Latin phrase meaning no more, as in no more can be done. It is perfectly correct to modify it with adverbs like completely or utterly, creating expressions such as completely nonplussed. In this sentence the adverb completely simply intensifies the feeling. The rest of the sentence, by the erratic behaviour of her father, clearly explains the cause. None of the alternative options offers a better or even grammatical expression, so the sentence should remain unchanged.

Step-by-Step Solution:

Verification / Alternative check:
A simple test is to remove completely and read the sentence as She was nonplussed by the erratic behaviour of her father. This still makes perfect sense and matches dictionary definitions of nonplussed. Adding completely simply increases the degree of surprise and confusion. On the other hand, there is no recognised word plussed in this context, and negative constructions like no-nonplussed are clearly artificial. Examples from respected writers and publications show similar patterns, such as He looked utterly nonplussed by the question. This supports keeping the original wording.

Why Other Options Are Wrong:
Option A, completely plussed, is incorrect because plussed is not used in standard English with the meaning of satisfied or unaffected; the word nonplussed is the established form. Option B, completely no-nonplussed, and option C, entirely not nonplussed, are clumsy double negatives that confuse rather than clarify the meaning. Option E, wholly un-nonplussed, is even more awkward and ungrammatical. None of these options is used in real English, and they all fail to convey the intended sense of deep bewilderment. Hence, No improvement is the correct answer.

Common Pitfalls:
A common misunderstanding is that nonplussed means calm, unaffected, or unimpressed, but that is a misuse in many varieties of informal speech. In standard exam English, you should remember that nonplussed means perplexed and unsure how to react. Another pitfall is trying to analyse the word into plus and minus and inventing opposite forms that do not exist. Instead, treat nonplussed as a single vocabulary item and learn its correct meaning and collocations. This will help you to select or reject similar options in vocabulary based questions with confidence.

Final Answer:
The sentence is already correct, so the right choice is: No improvement.