Final answer:
A Lewis acid is a substance that accepts a pair of electrons to form a covalent bond, which may include ions and neutral species with vacant orbitals. They are also known as electrophiles. The answer to the student's question is "C. Electron".
Step-by-step explanation:
A Lewis acid is defined as a substance that can accept a pair of electrons to form a covalent bond. According to the Lewis definition, a Lewis acid does not necessarily need to be a proton donor, unlike the Brønsted-Lowry definition which focuses on proton exchanges. A classic example of a Lewis acid is the hydrogen ion (H+), which is electron-deficient and can thus accept an electron pair. This also includes metal cations such as Mg2+ and Zn2+, and neutral species like boron trifluoride (BF3) and carbon dioxide (CO2). These species have vacant low-energy orbitals that enable them to accept electron pairs, fulfilling the role of electrophiles in chemical reactions. In contrast, Lewis bases are the donors of electron pairs, and are often referred to as nucleophiles because they 'love' nuclei or positively charged particles.