Incorrect spelling, explanation: this form is a mistake because fourth in 24th ends with th so we should use such an ending. rd is used with numbers ending with third, so for example 23rd. This shows that 24rd is an incorrect form, and the correct one is 24th.

Correct spelling, explanation: 24th is the correct form because suffix th is added to numbers which full-word form ends with th. Since twenty-fourth ends with th, 24th is the correct form. For the same reason, 24rd is incorrect. Suffix rd is used with numbers ending with rd, therefore for numbers including third (for example 33rd or 563rd).

Definition of 24th:
numeral, the ordinal number between 23rd and 25th
My favorite music shop is on the corner of St. Joseph’s and 24th Street. I highly recommend it.
Mrs. Jenkins is 94 years old and Timmy is her 24th grandchild!

Incorrect spelling

Incorrect spelling