Question

his greatest and a most remarkable achievement. is this sentence correct​

Answer 1

Answer:

Yes

Absolutely perfect

According to me in 1 way it's right though you insert 'a'

Answer 2

$\huge\boxed{\fcolorbox{red}{yellow}{Answer}}$

His greatest and a(the) most remarkable achievement.

$<b> Note :- </b>$

'the' will replace 'a'.