Final answer:
Phosphorus has 3 unpaired electrons in the 3p orbital with the configuration [Ne]3s2 3p3. Sulfur and chlorine have fewer unpaired electrons and argon has a full outer shell, thus no unpaired electrons.
Step-by-step explanation:
The element that has 3 unpaired 5p electrons is phosphorus (a). To understand why we need to look at the electron configuration of phosphorus which is [Ne]3s23p3. Phosphorus has 5 valence electrons, with three of those being in the 3p orbital. Since each of the three 3p orbitals can hold two electrons, the 3 unpaired electrons each reside in a separate 3p orbital, according to Hund's Rule, which states that electrons will fill an unoccupied orbital before they pair up.
Sulfur (b) has the electron configuration [Ne]3s23p4, so it has only two unpaired electrons in the 3p orbital. Chlorine (c) has one more electron than sulfur, so it has only one unpaired electron. Argon (d), being a noble gas with a full outer shell, has no unpaired electrons. Therefore, phosphorus is the correct answer to the question.