The complete sentence in present tense would be; Ella merienda frutas y queso.
What is the complete sentence?
The verb "merienda" is conjugated in the third person singular (ella) of the present tense and the nouns "frutas" and "queso" are in the plural form because they are both countable nouns.
So, we can change the sentence to the present tense as Ella está merendando frutas y queso or Ella se está merendando frutas y queso. These mean "She is having a snack of fruit and cheese" and "She is snacking on fruit and cheese" resectively.