Question

A group of scientists is studying the fossils of three different hominids. They find that the DNA of the first hominid is more similar to that of the second hominid than to that of the third hominid. Based on this observation, what scientific inference can the scientists make about these hominids?
The first and second hominids were able to mate and produce offspring, but the first and third hominids could not.
The first and second hominids shared the same geographic location, while the third was isolated.
The first and second hominids most likely became extinct before the third hominid species.
The first and second hominids have a more recent common ancestor than the first and third hominids.

Answer 1

"The first and second hominids were able to mate and produce offspring, but the first and third hominids could not." Although that might be the case, it might also not. "More similar" needn't be 'similar enough' (disregarding the fact that all homonid specimens involved could happen to share the same sex). 

"The first and second hominids shared the same geographic location, while the third was isolated." 

That information would be revealed by the geographic location they came from. 

"The first and second hominids most likely became extinct before the third hominid species." 

Perhaps or perhaps not. As ancestral lineages can survive for longer than descendant lineages, it needn't apply. Still, I suppose the non-existent law of averages provides some sort of justification for the use of "Most likely". 

"The first and second hominids have a more recent common ancestor than the first and third hominids." 

Assuming these are all in the same neat lineage, then the most recent common ancestor of the first and second is also the most recent common ancestor of the first and the third.

Answer 2

Answer:

The first and second hominids have a more recent common ancestor than the first and third hominids.

Explanation:

The similarity in DNA can definitely reveal us about common ancestry but it does not guarantee that the two species must be mating and reproducing. The best example to support this statement is that humans and chimpanzees  share a common ancestor and both have so much similarity in their DNA. The DNA of humans and chimpanzees is around 99% similar but still humans and chimpanzees cannot mate to produce off springs. Several attempts have been made in past to obtain fertile off springs by mating humans and chimpanzees but so far their mating has not produced any fertile off spring.