Incorrect spelling, explanation: the indicator nd is used with the number 2 to show the second number. It becomes clear when we will remember that these are the two last letters that occur at the end of the ordinal, for example, the first st added to 1, second nd to 2, third rd to 3, twenty-ninth th to 29, and so on. As a result, 29th, 2nd are the correct forms but 29nd is incorrect.

Correct spelling, explanation: 29th is the ordinal to which the indicator th is added. These letters occur at the end of the word twenty-ninth. 29nd does not exist because nd comes from the word second and is added only to 2. It shows that exclusively 29th is correct, whereas 29nd is wrong.

Definition of 29th:
number, one before 30th in a position or series,
Last night there was a fire on the 29th floor.
I will have been in Ruse by the 29th of January.

Incorrect spelling

Incorrect spelling