In the following sentence with a blank, choose the most appropriate word. He is not a native speaker yet he has a good __________ over German.

Introduction / Context:
This question tests knowledge of common English collocations related to language ability. The sentence describes a person who is not a native speaker but still has a strong ability in German. The blank must be filled with a noun that naturally combines with the preposition over in this context and accurately expresses language proficiency.

Given Data / Assumptions:
• The sentence is: He is not a native speaker yet he has a good __________ over German.• The person has high skill in German, despite not being a native speaker.• The options offered are authority, hold, command and expertise.

Concept / Approach:
In English, the standard collocation for high language proficiency is a good command of a language. Sometimes exams slightly vary the preposition to over, but the core collocation remains good command. Phrases like authority over German, hold over German, or expertise over German are not as natural or idiomatic. The word authority is more about power or influence, hold is about physical grip or control, and expertise is usually followed by in, not over, for subject areas.

Step-by-Step Solution:
Step 1: Identify the concept needed: language proficiency.Step 2: Recall typical expressions: good command of English, excellent command of French, etc.Step 3: Compare this with the options. Command fits directly into the familiar phrase good command.Step 4: Insert command into the sentence: He is not a native speaker yet he has a good command over German. This reads naturally as an exam sentence, even though many writers prefer command of German.Step 5: Conclude that command is the correct choice.

Verification / Alternative check:
Test the other options in natural sounding sentences. A good authority over German sounds odd, because authority normally refers to power or official position, not skill in a language. A good hold over German suggests control rather than proficiency and is not a common expression. A good expertise over German is clearly incorrect because the normal preposition is in, as in expertise in German. This series of checks confirms that command is the most appropriate word.

Why Other Options Are Wrong:
Option a: authority primarily means power to give orders or influence others. It does not naturally describe language skill.Option b: hold is used in phrases like have a hold on someone or hold over something, which do not fit the meaning of fluency or proficiency.Option d: expertise is usually followed by in, and even with in, expertise in German would refer to academic or technical expertise, not necessarily everyday fluency.

Common Pitfalls:
Because all four options are positive words, candidates may be tempted to choose based on general sense rather than collocation. However, English usage is strict in many fixed expressions, and exam questions often target these. Remembering classic phrases like command of a language and good command over English can help you quickly eliminate less idiomatic combinations. Always mentally place the option inside the sentence and ask whether you have seen or heard that exact phrase before.

Final Answer:
The natural expression for strong language proficiency here is good command. Correct answer: command.