71st

Correct spelling, explanation: 71st is an ordinal number and its full form is spelled seventy-first. It appears between seventieth and seventy-second. This spelling is correct because there is a rule in English according to which the first three numbers (one, two, and three) have an irregular way of creating ordinal forms. One changes to first, two – to second, and three – to third. When an ordinal number ends with 1, 2, or 3, we have to add a different suffix to its ending. Therefore, the only correct form is 71st, not 71th.

Definition of 71st:
numeral, coming next after the seventieth in position
I want to see you, but I have to visit my grandmother – it’s her 71st birthday.
I wonder what the world would look like in the 71st century.

71th

Incorrect spelling, explanation: this mistake is quite common because most ordinal numbers are created by adding the ending -th to a number – except for the first three numbers. 71 ends with one, so its ordinal form must be spelled seventy-first (71st). Although, we have to remember about the number eleven – its ordinal form is spelled eleventh (11th). 71th is an incorrect form because it doesn’t match its full spelling.

71nd

Incorrect spelling

71rd

Incorrect spelling