When caught off guard by the crash of thunder, 2-month-old diane arches her back,

The correct answer is:

Moro Reflex

Explanation:

According to neonatology, the moro reflex first described by the Austrian pediatrician Ernst Moro, corresponds to an infant automatic response from the motor nervous system that can be appreciated during the first three months of life approximately. The Moro Reflex response normally consists of three different steps in babies, in the first step they flail the arms outward, in the second step they quickly bring them back, and during the final step they generally cry.