Final answer:
The electron configuration 1s2 2s2 2p6 3s2 3p5 corresponds to the element chlorine, which has an atomic number of 17.
Step-by-step explanation:
Given the electron configuration 1s2 2s2 2p6 3s2 3p5, we can determine which element it represents by counting the total number of electrons and matching this to the atomic number on the periodic table. The given configuration has 2 electrons in the 1s subshell, 2 electrons in the 2s subshell, 6 electrons in the 2p subshell, 2 electrons in the 3s subshell, and 5 electrons in the 3p subshell. This adds up to 17 electrons, which corresponds to the element with the atomic number 17.
By consulting the periodic table, or with knowledge of the electron configurations of elements, we can conclude that the element with 17 electrons is chlorine (Cl). Thus, the correct answer to the student's question is c. chlorine, as its electron configuration is 1s2 2s2 2p6 3s2 3p5.