Answer: First habits and primary circular reaction
Explanation:
First habits are the initial habits that a child learns between the range of one-month to four month old.Primary circular reaction are the responses that accidentally occur and baby tends to repeat those movement according to their ability.E.g.- sucking thumb in mouth etc.
According to the question ,Rebecca is displaying primary circular reaction as initial habit that she gained accidentally by putting her thumbs in her mouth for sucking it and thus, repeats the action due to enjoyment.