Answer:
B.) When Framton saw the horrified look on Vera's face, he spun around to look out the window.
Explanation of why the others are incorrect:
A.) Framton saw the horrified look on Vera's face, so he spun around to look out the window: This uses a coordinating conjunction (so) to join two independent clauses, implying a cause-and-effect relationship between the two actions. However, the original sentences do not convey a clear cause-and-effect relationship, but rather a sequence of events.
C.) Framton saw the horrified look on Vera's face, and he spun around to look out the window: This uses a coordinating conjunction (and) to join two independent clauses, which gives equal importance to both sentences, implying that the two actions are unrelated or of equal importance, which may not convey the intended meaning of the original sentences. The original sentences suggest that the second action (spinning around to look out the window) is a response to the first action (seeing the horrified look on Vera's face), which is better conveyed using a subordinate conjunction such as "when" in option B.