Answer: D. bias
Step-by-step explanation:
The author of the article that was highly critical of President Truman's firing of General MacArthur once served under MacArthur. This means that he more likely than not, had some sort of relationship with MacArthur in which he looked up to the general.
In criticizing Truman therefore, there was a high probability that he did so because he took the side of MacArthur in the conflict and so was biased in his criticism of the President which is a factor that you should account for in your research.