In French, dates are always preceded by the masculine singular definite article le. The standard pattern is:
le + day (number) + month + year
For example: le 25 aout 1940, le 14 juillet 1789, le 1er janvier 2000.
This rule applies regardless of the month or context. The article does not change to la, les, or l' before a date because the implicit noun being modified is jour (day), which is masculine singular. The numeral "25" begins with a consonant sound, so there is no elision - it remains le, not l'.
The complete sentence reads: "Madame Nana est nee le 25 aout 1940."
