Final answer:
The common factor of the expression 1/9a + 1/9 is 1/9, making option 'a' the correct answer.
Step-by-step explanation:
The expression given is 1/9a + 1/9. To identify the common factor, we look for a term that is present in each part of the expression. Both terms have a 1/9 present, so the correct answer is 'a) 1/9'. This is because you can factor out the 1/9 from both terms, which would leave you with a + 1 after factoring. This process simplifies the expression while retaining its original value. It is essential to recognize such common factors to streamline mathematical expressions. In this case, the common factor 1/9 plays a key role in simplifying and understanding the structure of the given algebraic expression.