B. A possessive noun should have its instead of it's (which is a contraction for it is).
For A. Diana is a singular noun and the apostrophe should come after the r instead of after the s.
For C. The duck would've dunked ITS instead of IT'S beak since it is possessive.
For D. You do not need to put an apostrophe since theirs is already a possessive noun.
The answer is B...knowing about her heart condition develops her character, this in turn makes her later death believable because she has been characterized as someone with a fragile heart.
The answer would be B, not every run on sentence has two complete thoughts.