The correct answer is: A) Coastal trade.
Shell ornaments are often found in archaeological sites along coastal areas. This suggests that people in these areas were involved in coastal trade. Shells were a valuable commodity that could be traded for other goods. They were also used to make jewelry and other decorative items.
The other options are not as likely explanations for the use of shell ornaments in prehistoric Kerala. Religious symbolism is possible, but there is no evidence to support this claim. Limited resources is also possible, but it is more likely that people would have used other materials, such as stone or bone, if they were limited in their resources. Aesthetic preferences is also possible, but it is more likely that people would have used other materials, such as feathers or fur, if they were primarily interested in aesthetics.
In conclusion, the use of shell ornaments in prehistoric Kerala suggests that people in these areas were involved in coastal trade. Shells were a valuable commodity that could be traded for other goods. They were also used to make jewelry and other decorative items.