Incorrect spelling, explanation: this form is incorrect because we add suffixes to ordinal numbers according to how full-word forms end. So, sixth ends with th, so the number form will be 6th. –st would be used with number first, for example, 91st. Therefore 6st is incorrect and the correct form is 6th.

Correct spelling, explanation: 6th is the correct form, because the full word is sixth, therefore the number form ends with th. 6st is mistaken because it would end with st, which is not the case: only first ends with st, so all numbers with first at the end will have this suffix. Therefore 6th is the only correct form.

Definition of 6th:
numeral, the ordinal number between 5th and 7th
On Jack’s 6th birthday there will be a clown and a magician.
He has got really good grades – he is actually the 6th best student in his school.

Incorrect spelling

Incorrect spelling