Which of the following describes a characteristic of nucleophiles?

Nucleophiles are defined primarily as electron-rich species that are capable of donating an electron pair to an electrophile in a chemical reaction. This characteristic is fundamental to their role in organic reactions, particularly in nucleophilic substitution and addition processes. Being electron-rich allows nucleophiles to attack positively charged or electron-deficient species, which are often electrophiles. Thus, this defining characteristic of nucleophiles aligns with the answer provided.

In contrast, nucleophiles do not necessarily possess a partial positive charge, as being electron-rich typically corresponds to having either a negative charge or having lone pairs of electrons available for donation. While some nucleophiles might be larger, bulkiness is not a defining characteristic, as smaller nucleophiles can also be very effective. Additionally, nucleophiles are not limited to elimination reactions; they often participate in a variety of organic reactions beyond just elimination, including substitution and addition reactions. This makes the description of nucleophiles as exclusively participating in elimination reactions inaccurate.