Answer: 
The correct answer would be option c. Y chromosome. 
In humans, the males contain one X chromosome and one Y chromosome. In contrast, females contain two X chromosomes. 
Thus, DNA sampling of the Y chromosome would be best to determine if the two male samples are related or not. 
In addition, mtDNA is often inherited from the mother and thus, it would not be a good option to determine the relationship between the male samples.