How does losing an electron result in a positive charge?

A proton has a positive charge, an electron has a negative charge. Usually, the amount of electrons and protons is the same, but when an electron is lost usually because of radiation or a chemical reaction the resulting amount of electrons/ protons results in a different charge for the atom in general.
When there are more electrons the result it a negative ion, and when there are more protons it is a positive ion. 
So, to answer your question: if an electron is lost from a neutral atom and there are more protons than electrons you get a positive charge. Have a good day!

Julia would be observing a. static electricity was caused by her sweater rubbing against her shirt and transferring electrons. she didn't touch anything metal, so that would cancel out b.she most likely didn't rub her sweater against her hair either, so that cancels out c.she also didn't scoot across the carpet while trying to take her sweater off, that cancels out d.so that leaves a.a. is correct.