29th

Correct spelling, explanation: in English, there are some indicators that say which the ordinal number is. Th is added to the numbers 4, 5, 6, etc., and nd only to the number 2, meaning the second position of the object. This indicator should occur with 2 because we use two letters from the end of the ordinal. For example, in the word second there are the letters nd that we add to 2 but in twenty-ninth th appears at the end. It means that th is added to 29. To sum up, 29th is correct, 29th is incorrect.

Definition of 29th:
the ordinal number between 28th and 30th,
The 29th of February is a leap year and the next will be in 2024.
What happened on the 29th of July?

29nd

Incorrect spelling, explanation: the learners of English can confuse the indicators such as st, nd, rd, and th that are added to the numbers in figures in order to make the ordinals. In fact, nd should be added to number 2 indicating the second, where two letters nd occur at the end. However, the number 29th is twenty-ninth and the last two letters are th, therefore, 29th is error-free form and 29nd is wrong.

29rd

Incorrect spelling

29st

Incorrect spelling