Final answer:
The correct past participle of the verb 'abrir' is 'abierto.'
Step-by-step explanation:
The correct form of the past participle of the verb 'abrir' is 'abierto.' In this case, since the adjective 'abierto' refers to 'la puerta,' which is a singular feminine noun, we use the feminine singular form. If we were talking about multiple doors, we would use the plural form 'abiertos' for masculine nouns or 'abiertas' for feminine nouns.
Learn more about Spanish verb conjugation