Why is "El este o savant, ea este o cercetatoare" wrong?
Because savant is masculine, so you need to use un as the article in front of it.
When you put your cursor over "a" it shows o as the first choice. The answer doesn't even say un or o cercetatoare.
That's because we usually omit the indefinite article when specifying someone's profession. The variant with "un" and "o" is less common, but also correct.