Final answer:
The correct word to complete the sentence is 'Ladies', which is option a). It is the plural form of 'lady' and is used to address multiple women in a formal setting like a circus announcement.
Step-by-step explanation:
The correct form of the word to complete the sentence "Ladies and gentlemen and children of all ages, the circus is about to begin." is option a) Ladies. This term is the plural form of 'lady' and does not possess anything, so the possessive forms 'lady's' and 'ladies'' are incorrect.
Additionally, the plural possessive 'ladie's' is not only possessive but also incorrectly spelled, so it is not suitable for this context either. When addressing a group that includes multiple women, 'Ladies' is the appropriate word to use. In the context of this popular circus greeting, it sets a formal and welcoming tone for the announcement.