Final answer:
Based on Nader's understanding of spitting and not swallowing toothpaste, I would guess his age to be around 2 years.
Step-by-step explanation:
Based on the question, it seems like the student is being asked about a child's understanding of dental hygiene. The student is being asked to guess Nader's age based on his understanding of spitting and not swallowing toothpaste. Typically, children between the ages of 2 and 4 start to understand and have the ability to spit out toothpaste rather than swallowing it. Therefore, I would guess the student's age to be b) 2 years.