Answer:
The correct abbreviation is M. et Mme Dupont. Note that there's a space between "Mme" (Mme Blanc) and the last name and only a dot between "M." and the last name (M.Gras)
Step-by-step explanation:
In French, we use M. for Monsieur, Mme for Madame and Mlle for Mademoiselle (even though we don't use this one anymore, just like Miss). If there are several men, you can use the form MM.Dupont et Boucher.