Answer: Option 'c' is correct.
Step-by-step explanation:
Since we have given that
Average number of arrivals = 6 per hour
Service rate = 3 every 15 minutes
Service rate is given by
[tex]\mu=\dfrac{60\times 3}{15}=12[/tex]
So, the average number of customers in the system is given by
[tex]\dfrac{\mu}{\mu-\lambda}\\\\=\dfrac{6}{12-6}\\\\=\dfrac{6}{6}\\\\=1[/tex]
Hence, Option 'c' is correct.