Which of the following is the correct Spanish representation of the number 21?

The number 21 in Spanish is represented as "veintiuno." This term combines "veinte," which means twenty, with "uno," indicating one more than twenty. In the Spanish counting system, numbers from 21 to 29 are generally formed by taking the base number "veinte" and adding the units from one to nine. Therefore, "veintiuno" clearly signifies twenty-one.

The other choices represent different quantities: "treinta" is thirty, "veintidós" translates to twenty-two, and "veintiséis" means twenty-six. Each of these reflects a different number, illustrating how Spanish numerals are constructed, but none of them corresponds to twenty-one.