The answer is ribose.
mRNA is actually messenger ribonucleic acid. As any other RNA (ribonucleic acid) it consists of a phosphate group, sugar ribose, and nucleobases (adenine, uracil, cytosine, guanine). So, it is called RIBOnucleic acid because it consists of sugar RIBOSE.
Take for example DNA - deoxyribonucleic acid. It is called DEOXYRIBOnucleic acid because it consists of sugar DEOXYRIBOSE.