Which of the species is not paramagnetic?

The correct option is Cl^-.

To determine which species is not paramagnetic, we need to look at their electron configurations and count the number of unpaired electrons:
1. Paramagnetic species have one or more unpaired electrons and are attracted by an external magnetic field.
2. Diamagnetic species have all of their electrons paired (zero unpaired electrons) and are weakly repelled by a magnetic field. Thus, a species that is "not paramagnetic" must be diamagnetic.

Let's write down the electron configurations for each of the given species:

1. Chloride ion (Cl^-):
Chlorine (Cl) has an atomic number of 17. Its neutral ground-state electron configuration is:
1s² 2s² 2p⁶ 3s² 3p⁵
When neutral chlorine gains one electron to form the chloride ion (Cl^-), the electron configuration becomes:
1s² 2s² 2p⁶ 3s² 3p⁶
In this configuration, all subshells are completely filled, meaning every electron is paired. The number of unpaired electrons is:
$n=0$
Therefore, Cl^- is diamagnetic (not paramagnetic).

2. Beryllium ion (Be⁺):
Beryllium (Be) has an atomic number of 4. Its neutral ground-state electron configuration is:
1s² 2s²
When it loses one electron to form Be⁺, its configuration becomes:
1s² 2s¹
The single electron in the 2s orbital is unpaired:
$n=1$
Therefore, Be⁺ is paramagnetic.

3. Neon ion (Ne²⁺):
Neon (Ne) has an atomic number of 10. Its neutral ground-state electron configuration is:
1s² 2s² 2p⁶
When it loses two electrons to form Ne²⁺, its configuration becomes:
1s² 2s² 2p⁴
According to Hund's rule, the 2p⁴ configuration contains one paired set of electrons and two unpaired electrons in the 2p orbitals:
$n=2$
Therefore, Ne²⁺ is paramagnetic.

4. Arsenic ion (As⁺):
Arsenic (As) has an atomic number of 33. Its neutral ground-state electron configuration is:
[Ar] 3d¹⁰ 4s² 4p³
When it loses one electron to form As⁺, its configuration becomes:
[Ar] 3d¹⁰ 4s² 4p²
According to Hund's rule, the two electrons in the 4p subshell occupy separate orbitals with parallel spins, meaning there are two unpaired electrons:
$n=2$
Therefore, As⁺ is paramagnetic.

Conclusion:
Since Cl^- has a closed-shell noble gas configuration with zero unpaired electrons, it is diamagnetic, making it the only species listed that is not paramagnetic.