Which of the following are types of questions that are useful in opening and testing...

In the opening phase, parties should confirm understanding and get the issues on the table.

The testing phase is an information gathering stage where the hypothesis and assumption you have made in the planning stage can be tested or confirmed or disproved.

Opening questions (those that start with 'what', 'how', 'why') are used at the opening and testing stages to uncover needs and underlying motives, and to allow the buyer to get a feel of what is in store in the negotiation.

Probing questions are also useful to check that the supplier fully understand their offering, as well as your needs, and can also be used to communicate to the supplier that you know this category well. These questions are typically useful at the opening and testing stages.