This is only the most tenuous evidence for it.

The correct answer is: D. less

Tenuous means “not strong or firm”. So, the most tenuous evidence is the least strong or firm evidence.

Abundant means “existing in large quantities”. So, abundant evidence is a lot of evidence.

Enough means “as much or as many as is needed or wanted”. So, enough evidence is the amount of evidence that is needed or wanted.

Reasonable means “based on or in accordance with reason”. So, reasonable evidence is evidence that is based on reason.

In this case, the evidence is not strong or firm, so it is not abundant, enough, or reasonable. It is, therefore, less.