Vendeur/vendeuse is specifically covered in the lesson with an example. The answer to the rest of the question relies on extending from the other information given in the lesson:

1. The profession name is in the plural, it ends in an 's' (whereas neither of the singular forms end in 's').

2. (Une) Vendeuse is the feminine noun in the singular, it takes an end 's' in the plural. The plural feminine form of the noun can ONLY apply to an all female 'group'. Therefore the only correct answer among those offered is Grace and Anna.  

3. Un vendeur is the masculine singular noun - the plural « vendeurs » is used for BOTH an all-male or a mixed group. If the sentence was « Vous êtes vendeurs » you would need more detail to know whether it was Joseph and Tom, or Grace and Tom.