IS or ARE

[mamajonov]

According to the writings of [url]Thorstein Veblen, the economist, the most reliable signal of a truly wealthy individual is his or her ability and willingness[/url] to engage in "conspicuous consumption""”to spend it in a way that is patently absurd or irrational.
E. the economist Thorstein Veblen, the most reliable signal of true wealth is an individual's ability and willingness

Dear Experts,
ALthough I understand that choice E is best option among others, and is correct answer, I do not uderstand how can we use "is" for "individual`s ability and willingness". In my understanding the correct phrase should be "most reliable signals of true wealth are an individual`s ability AND willingness".
Could you please explain why choice E is correct?
Thanks

[RonPurewal]

The subject of "is" is "signal".

If two ideas are closely tied together, then the use of "it" for the combination of both ideas is commonplace. E.g., #82 in the 12th edition OG, in which "it" stands for "owning and living..." -- two things considered together as a single goal, not as two separate goals.
Same thing here. Ability and willingness can logically be represented as one idea.

[mamajonov]

Thanks for the reply.

[RonPurewal]

You're welcome.