Final answer:
The least effective or unavailable response to cool conditions in newborns is 'unflexing from the normal position' because it does not contribute significantly to heat conservation or production, unlike nonshivering thermogenesis which metabolizes brown fat.
Step-by-step explanation:
To address the question on which infant response to cool environmental conditions is either not effective or not available to them, let's consider each option presented: constriction of peripheral blood vessels, metabolism of brown fat, increased respiratory rates, and unflexing from the normal position.
Newborns have an underdeveloped nervous system and immature musculature, which limits their ability to effectively constrict blood vessels and generate heat by shivering, responses that would be helpful in an adult in combating cold. However, they do have brown fat and utilize nonshivering thermogenesis as an effective method for generating heat. Given these points, and considering that increased respiratory rates would not generate significant heat to combat cold conditions, the less effective response available to infants would be 'unflexing from the normal position' since this does not contribute significantly to heat conservation or production.