Final answer:
Based on the descriptions given, the person is able to maintain balance and has basic balance abilities, but the ability to cross midline cannot be determined without specific information showing that action.
Step-by-step explanation:
The ability of the person described in the given content seems to involve maintaining balance and station. Regarding the options provided and the referenced descriptions of body positions, it can be determined that the person is able to sit unsupported and able to weight shift, showing basic balance abilities. Specifically, the description of standing with upper limbs extended at a 90-degree angle and the lower limbs in a wide stance suggests active muscle control to maintain balance in a standing position. The ability to cross midline, however, cannot be determined from the given descriptions as it refers to the ability to reach across the body's midline with arms or legs, which isn't described in the content provided.
From the given descriptions in the content, letter (b) and letter (c) suggest different positions that the person is able to achieve (standing in a particular position and lying supine). The content relating to unstable equilibrium and center of gravity also supports that the person is using muscles for balance and has achieved a certain level of postural control. Nevertheless, the information provided does not specify a clear test or observation of the person's ability to cross the midline. Therefore, cannot be determined is the most appropriate answer based on the options.