Deep sunken valleys suitable for mussel culture are known as:

Correct option is B

Correct Answer: (b) Rias
Explanation:
→ Rias are drowned river valleys with steep sides and good water exchange — ideal for mussel farming because they provide sheltered deep water and nutrient flows.
→ Many successful mussel farms (e.g., in Galicia, Spain) are located in rias.
Information Booster:
→ Fjords are glacially carved deep valleys (also used for aquaculture) but rias are the common term for sunken river valleys suited to mussel rafts/longlines.
Additional Information (Other Options):
(a) Parks: unrelated.
(c) Fjords: glacial origin (deep but different formation).
(d) Bar-built: lagoons formed by sandbars (shallower).