Which of the following technique is useful in patients with Trismus?
Typically, the occlusal radiograph is especially useful in the following cases:

To locate precisely roots and supernumerary, unerupted, and impacted teeth (this technique is especially useful for impacted canines and third molars).
To localize foreign bodies in the jaws and stones in the ducts of sublingual and submandibular glands.
To demonstrate and evaluate the integrity of the anterior, medial, and lateral outlines of the maxillary sinus.
To aid in the examination of patients with trismus, who can open their mouths only a few millimeters; this condition precludes intraoral radiography, which may be impossible or at least extremely painful for the patient.
To obtain information about the location, nature, extent, and  displacement of fractures of the mandible and maxilla.
To determine the medial and lateral extent of disease (e.g., cysts, osteomyelitis, malignancies) and to detect disease in the palate or floor of the mouth.

Ref: ORAL RADIOLOGY Principles and Interpretation,Stuart C. White,Michael J. Pharoah Edition 7 page no 126