Answer: is utilizing secondary circular reactions
Explanation: Secondary circular reactions are the reaction that defines the sub-stage of the baby whose age is in the range of 4 months- 8 months . The child tends to repeat the activity with some action or object that makes them feel pleasurable and gain the response from surroundings.
According to the situation mentioned in the question, Felena is also indulging in the activity of using the secondary circular reactions from rattle which makes understand that the rattling sound is coming from it.