Choose the best alternative to complete the given sentence from the options given below.

They are having trouble _________ the contract.

nailing down

The best alternative to complete the given sentence is: ☀ nailing down

The sentence would be: "They are having trouble nailing down the contract." Explanation: The phrase "nailing down" is an idiomatic expression that means to finalize or secure something, in this context, the contract. It suggests the difficulty they are facing in coming to a final agreement or resolution regarding the contract.