Sentence Correction

Critics fear that the decline in what constitutes a good secondary school education have weakened this country's standing in the world.

A) in what constitutes a good secondary school education have

B) in what constitutes a good secondary school education has

C) in what constituted a good secondary school education has

D) in what constituted a good secondary school education have

E) in constituting a good secondary school education has


OAB

I got this one right.

My doubt : what is the use WHAT in this sentence?

Please advise

HiExperts ,

Can you shed some light on it?

Thanks,

Shreyans