Answer:
a. Henry and Anne, who have just had their first baby.
Step-by-step explanation:
From the options listed, the best option would be that the couple would be the happiest due to the birth of their first baby. The reason behind that is Happiness is like a state and not a trait, happiness isn't long lasting but can change quickly. Here, the birth if the baby will surely give to much happiness and joy to the couple but who knows how long it may last.
Happiness is a satisfaction and it is evident whenever a couple is blessed with their first baby, the satisfaction is real and it feels like a long due task has been achieved of life.
Hence, In my view, option 'a' is the correct answer.
Hope this helps.
Goodluck and thanks buddy.