Which of the following sentences is grammatically correct?

The correct answer is D. This sentence is grammatically correct as it uses the conjunction 'because' to connect two clauses properly. 'Because the patient could walk' is a dependent clause, and 'he was allowed to leave his room' is an independent clause. Option A is incorrect because it lacks a conjunction to connect the two subjects correctly. Option B is a sentence fragment without a main clause. Option C is incorrect due to the misuse of commas to separate the subjects; it should use 'and' to list the subjects properly. Therefore, Option D is the only grammatically correct sentence among the choices.