Tap M fills 5 buckets in 40 minutes ⇒ $\frac{40}{5}=8 \mathrm{minutes}$

Tap N fills 7 buckets in 49 minutes ⇒ $\frac{49}{7}=7 \mathrm{minutes}$

Tap O fills 1 bucket in 30 minutes ⇒ $\frac{30}{1}=30 \mathrm{minutes}$

In 3 hours,

Number of buckets filled by tap M ⇒ $\frac{180}{8}=22.5 \mathrm{buckets}$ 

Number of buckets filled by tap N ⇒ $\frac{180}{7}=25.7 \mathrm{buckets}$ 

Number of buckets filled by tap O ⇒ $\frac{180}{30}=6 \mathrm{buckets}$ 

Total number of buckets filled ⇒  (22.5 + 25.7 + 6) = 54.2 buckets

Total capacity of the tank = Total number of buckets filled X Capacity of each bucket

 ⇒  54.2 x 6 = 325.2 L

Hence option B is correct.