The word which fills in the blank is an adjective as it describes the subject, "our teacher". The clue lies in the words, "She easily captivates the attention of the students". These words describe the teacher. Let us look at the meaning of each word in the given options.
The word, "dynamic" is an adjective which describes a person "having a positive attitude and full of energy and new ideas". A person full of energy and new ideas can easily captivate the attention of students through his/ her energetic conversations. This word most appropriately describes the teacher who "captivates the attention of the students". Hence, option B is correct.
Option A: The word, "lethargic" is an adjective which means "sluggish and lacking energy". A person who is sluggish and lacks energy cannot easily captivate the attention of students. Hence, this is incorrect.
Option C: The word "iconic" is an adjective which means "very famous or popular, especially for his/ her distinctive excellence in a particular field". For example: "He is an iconic writer". This means, he is a famous writer who has achieved a certain level of excellence. This is quite different in meaning and does not describe a person who can easily captivate the attention of students. Hence, this is incorrect.
Option D: The word, "uninteresting" is the opposite of the word "interesting". It is an adjective which means not arousing curiosity or interest. A person who is uninteresting cannot easily captivate the attention of students. Hence, this is incorrect.