Silica sand, an important ingredient in glassmaking, is found in:

The correct answer is: b) Alappuzha.

Silica sand is found in abundance in Alappuzha district of Kerala. It is an important ingredient in glassmaking and other industries. The sand is extracted from the river beds and is then processed to remove impurities. It is then used in the manufacture of glass, ceramics, and other products.

The other options are incorrect because:

• Palakkad is a district in Kerala that is not known for its silica sand deposits.
• Idukki is a district in Kerala that is known for its tea plantations and hydroelectric projects.
• Thiruvananthapuram is the capital of Kerala and is not known for its silica sand deposits.