The following sentence has been divided into parts.(A) While teaching, / (B) she draws the diagrams very careful / (C) on the board.

Explanation:

The error is in part (B): “she draws the diagrams very careful”
 The word “careful” is an adjective, but here we need an adverb because it is modifying the verb “draws”.

✔ Correct word: carefully (adverb)
Correct sentence:

While teaching, she draws the diagrams very carefully on the board.

Why other parts are correct:

• ✔ (A) “While teaching,” → correct phrase
• ✔ (C) “on the board.” → correct prepositional phrase

Extra Facts (Important for Exams):

📌 Adjective vs Adverb Rule:

• Adjective → describes noun (careful person)
• Adverb → describes verb (draw carefully)

👉 Common mistake:

• ❌ She speaks careful
• ✔ She speaks carefully