Final answer:
The subject that completes the sentence is 'Sus hijos' (Their children).
Step-by-step explanation:
The subject that correctly completes the sentence is Sus hijos.
'Pueden cuidar a la abuela' translates to 'They can take care of the grandmother'.
In this sentence, the subject 'They' refers to multiple people, so the correct option is 'Sus hijos', which means 'Their children'.
Learn more about Subject completion in a Spanish sentence