The correct answer is (1) Chemical
Step-by-step explanation:
A compound is the result of two or more chemical elements that bond to form new substances; for example, water is a compound because this contains two hydrogen atoms bonded to one oxygen atom. Moreover, compounds are the result of a chemical change that occurs as two or more substances interact in a chemical reaction, and this leads to the formation of bonds between the substances or the formation of compounds.
On the other hand, a physical change involves only a change in the state of the matter and a nuclear change leads to a change in the atom rather. Thus, the type of change that must occur to form a compound is a chemical change.