When performing neurological reflexes on the infant, which primitive reflex will be present longest?

Babinski  sign.

Explanation :

Except for the Babinski, which disappears around 1 year of age, these primitive reflexes diminish over the first few months of life, giving way to protective reflexes.

Primitive reflexes are subcortical and involve a whole-body response. Selected primitive reflexes present at birth include Moro, root, suck, asymmetric tonic neck, plantar and palmar grasp, step, and Babinski.