Learn Spanish (Spain): Mis abuelos son muy felices.

Usually, we use the verb estar for feelings because they change. However, when you use the verb ser (like son here) with feliz, it means they are naturally happy people or have a generally happy life. If you used están, it would mean they are feeling happy right at this specific moment.

Why is the plural spelled felices with a c instead of a z?

In Spanish, when a word ends in the letter z, you must change the z to a c before adding -es to make it plural. So, the singular word feliz becomes felices to match the plural subject Mis abuelos.

Does Mis abuelos mean my grandfathers or my grandparents?

It almost always means your grandparents. In Spanish, a mixed group of males and females uses the masculine plural form. Unless you are specifically talking about two men, abuelos translates to grandparents.

Why is the word for my written as Mis instead of Mi?

Possessive words must match whether the noun they describe is singular or plural. Because abuelos is plural, you have to use the plural form Mis.