The correct answer is b) Palakkad.
Limestone deposits are found in the Palakkad district of Kerala. The district is located in the central part of the state and is known for its rich mineral resources. Limestone is a sedimentary rock that is formed from the accumulation of calcium carbonate. It is a major raw material for the production of cement, lime, and other building materials. The limestone deposits in Palakkad are of high quality and are used in a variety of industries.
The other options are incorrect. Idukki is a district in the northern part of Kerala. It is known for its tea plantations and hydroelectric projects. Wayanad is a district in the northern part of Kerala. It is known for its forests and wildlife sanctuaries. Malappuram is a district in the northern part of Kerala. It is known for its textile industry.