The completed sentence is: "Tú tienes una mancha en tu cara.
How to write the sentence
In the sentence "Tú tienes una mancha en tu cara," the correct word to fill in the blank is "tu," which is a possessive adjective in Spanish meaning "your."
"Tu" is used before a singular feminine noun like "cara" (face) to indicate possession or ownership. In this case, it signifies that the spot or stain (mancha) is on your (tu) face (cara).
Possessive adjectives in Spanish, such as "tu," agree in gender and number with the noun they modify.
Since "cara" is singular and feminine, the corresponding possessive adjective is "tu" to match and indicate possession of something related to the person being addressed.