Final answer:
All listed transducers (conventional straight-beam, immersion, EMAT, and magnetostrictive) are capable of generating guided waves in their standard applications. Hence, none of these options can be singled out as not able to generate guided waves without additional context or specific conditions laid out in the question.
Step-by-step explanation:
The student asked which type of transducers cannot be used to generate guided waves. The types of transducers that are capable of generating guided waves include conventional straight-beam transducers, immersion transducers, EMAT (Electromagnetic Acoustic Transducers), and magnetostrictive transducers.
Conventional straight-beam transducers are often used in materials testing and can induce wave modes that travel along structures. Immersion transducers are typically used in applications where the transducer is immersed in a fluid, and they can excite various types of guided waves in materials. EMAT transducers generate ultrasonic waves via electromagnetic fields and can be used to excite guided waves without the need for couplant since they do not require direct contact with the material. Magnetostrictive transducers also utilize magnetic fields to generate waves and can be used for guided wave applications.
All of these transducers are designed to generate waves that can propagate through a medium in guided modes, so none of them fits the criteria of being unable to generate guided waves. Therefore, it could be that there is either a misunderstanding in the question, or perhaps an additional context that specifies certain conditions under which some of these transducers could not be used.