Select the most appropriate option to fill in the blank.

The teacher was very ______ in his subject.

proficient

The most appropriate option to fill in the blank is proficient.

The word "proficient" means "having a good knowledge or skill in a particular area". It is the most accurate word to describe a teacher who is very knowledgeable and skilled in their subject.

The other options are not as accurate.

• Top means "at the highest level". It could be used to describe a teacher who is the best in their field, but it is not as specific as "proficient".
• Master means "having complete mastery of a subject". It is a more extreme word than "proficient", and it is not always accurate to describe a teacher.
• Best is a general word that means "very good". It is not as specific as "proficient" and it does not necessarily mean that the teacher is knowledgeable and skilled in their subject.