Final answer:
Clay has negative charges that can attract and hold positively charged ions, known as cations. Hence, the correct answer to the student's question is (C)'Negative; cations'.
Step-by-step explanation:
The student's question about the type of charges on clay and the kind of nutrients it can attract can be answered with knowledge of ions and ionic bonding. In an ionic bond, oppositely charged ions are held together by an electrostatic force, which typically involves a cation with a positive charge and an anion with a negative charge. Clay particles have a net negative charge and thus tend to attract and hold onto positively charged ions, which are known as cations. Therefore, the correct answer is: c) Negative; cations.