To determine which substance among the given options is an acid, we should examine the chemical properties of each one. Here are the steps:
1. Be(OH)[tex]\(_2\)[/tex]: This is beryllium hydroxide. Hydroxides of metal elements are generally bases because they can accept protons (H[tex]\(^+\)[/tex]) and release hydroxide ions (OH[tex]\(^-\)[/tex]). Therefore, Be(OH)[tex]\(_2\)[/tex] is a base.
2. HCl: This is hydrochloric acid. It is a well-known and strong acid that dissociates completely in water to release hydrogen ions (H[tex]\(^+\)[/tex]) and chloride ions (Cl[tex]\(^-\)[/tex]). Therefore, HCl is an acid.
3. LiBr: This is lithium bromide. It is a salt formed from the neutralization of lithium hydroxide (a base) and hydrobromic acid (an acid). Since it is a salt and does not release H[tex]\(^+\)[/tex] ions on its own, it is neither an acid nor a base.
4. NH[tex]\(_3\)[/tex]: This is ammonia. It is a weak base because it can accept a proton (H[tex]\(^+\)[/tex]) to form ammonium ion (NH[tex]\(_4^\+\)[/tex]). Therefore, NH[tex]\(_3\)[/tex] is a base.
Given the chemical nature of each substance, we conclude that the acid in the list is:
[tex]\[
\boxed{HCl}
\][/tex]
