* Halogens are elements in Group 17 of the periodic table (like fluorine, chlorine, bromine, iodine). They have a high electronegativity, meaning they have a strong tendency to attract electrons.

* Gaining an Electron: To achieve a stable electron configuration (like the noble gases), halogens readily gain one electron. This forms a negatively charged ion called an anion.

Let's look at the other options:

* b. losing positive protons: Protons are located in the nucleus of an atom and determine its atomic number. Losing protons would change the element entirely, not just form a compound.

* c. losing positive protons: Same explanation as above.

* d. joining with both calcium and carbon: While halogens can form compounds with calcium and carbon, this isn't the defining characteristic of their bonding.

In summary: Halogens are highly electronegative and gain an electron to form negative ions (anions) when forming compounds.