The correct option is C To conceal my plan
An idiom or an idiomatic expression is a type of informal English that has a different meaning from its literal one.
The option with the opposite meaning is the correct one.
The idiom 'show your hand' means 'to reveal plans that were once kept secret'.
Option A: 'to yield' means 'to surrender', to 'give away to demands or pressure'. It's not opposite in meaning to the given idiom.
Option B: 'To shake hands' literally means to shake hands with another. It is not related to the idiom and hence, it isn't opposite in meaning.
Option D: 'To lose my ground' means 'to lose power or influence over something/someone'. It isn't related and hence, not opposite to the given idiom.
Option C: 'To conceal my plan' means 'to hide it', 'not to expose my plans'. This is opposite to the meaning of the idiom. Hence C is the correct option.