Step 1: Define a landlocked country.
A landlocked country is one that does not touch any ocean, so all its borders are shared with other countries or, in rare cases, an inland sea.
Step 2: Check each option against this definition.
China has a coastline on the Pacific Ocean, so it fails the landlocked test straight away. Switzerland is landlocked but covers only around 41,000 square kilometres, far too small to top this list.
Step 3: Compare the remaining landlocked giants.
Kazakhstan spans close to 2.7 million square kilometres in Central Asia, while Mongolia covers about 1.6 million square kilometres. Kazakhstan is clearly larger.
Final Answer:
Kazakhstan is the largest landlocked country in the world, so option C is correct. \[ \boxed{\text{Kazakhstan}} \]