Answer: The correct answer is they form an ionic bond by exchanging an electron.
Step-by-step explanation: There are 2 types of bond that can be formed between the elements.
1. Ionic bond: These bonds are formed when there is a complete transfer of electrons from one element to another. The element which donates an electron is a n electropositive element and an element which accepts an electrons is an electronegative element.
2. Covalent bond: These bonds are formed when there is sharing of electrons between two elements.
Lithium is the 3rd-element of the periodic table having electronic configuration:

This element can easily loose 1 electron.
Fluorine is the 9th-element of the periodic table having electronic configuration:

This element can easily accept 1 electron.
Hence, these elements form ionic bond because there is complete transfer of electrons from lithium to fluorine. As, only 1 electron is getting transferred from lithium to fluorine. Therefore, the correct answer is they form an ionic bond by exchanging an electron.