Study Curve

My best guess...

More Acidic/Less Basic: donates a hydrogen ion (H+)
More Basic/Less Acidic: accepts a hydrogen ion (H+)

A) Carboxyl group (R-C=O): The oxygen atom is very electronegative, which means that the electrons spend more time around it. This gives it a slightly negative charge. Thus, it could attract/accept a positively charged proton (H+). However, oxygen likes its negative charge and has no strong desire to get rid of it. So a carboxyl group would make something only slightly more basic.

B) Sulfhydryl group (R-SH): Sulfur (S) is very electronegative like oxygen. It likes to have negative charge around it. It will have no problem losing its proton. Since it can donate a proton, it would make something more acidic.

C) Amino group (R-NH2): The key thing to remember is that nitrogen has one lone pair of electrons when it bonds 3 atoms. This partially negative lone pair can and will accept positive hydrogen ions. Nitrogen is very happy with the extra positive charge, so adding it to a compound would make it much more basic.

D) Phosphate group (R-O-P(=O)-(OH)2): Oxygen is very electronegative. It likes to have negative charge around it. The hydroxyl groups (-OH) on the phosphate will have no problem losing a proton. Since they can donate a proton, a phosphate would make something more acidic.

This should be a carboxyl because it is an oxygen group which is more alkaline. But it's been a while.