Final answer:
Glycine is the only amino acid without a chiral center among the options given, due to its two hydrogen atoms attached to the alpha carbon, making it symmetrical.
Step-by-step explanation:
The amino acid without a chiral center is glycine. Unlike alanine, serine, and valine, glycine's alpha carbon (a-carbon) is not a chiral center because it has two hydrogen atoms attached, making it symmetrical.
Therefore, glycine cannot exist in enantiomeric forms and does not rotate plane-polarized light. The chemical nature of the side chain determines the amino acid's properties, such as being acidic, basic, polar, or nonpolar. In the case of glycine, its side chain is simply a hydrogen atom, which contributes to its non-chiral nature.