Complete the following by using appropriate articles:

My office is on ________ sixteenth floor, and I usually take _____ office elevator. But today, I will be using ______ stairs. 

The English language is distinguished by three articles, and they can be classified into definite and indefinite articles. While “the” is regarded as the definite article, “a” and “an” are considered indefinite articles. “A” usually precedes a consonant(A ball, A pharmacy) or a vowel with a consonant sound(Eg. A yak, A university), and “an” is usually succeeded by a word that begins with a vowel(Eg. An astronaut) or a silent “h”(Eg. An hour).
“The” is used when we talk of a particular place, person, or thing and when we mention a singular noun representing the whole class(Eg. The heartbeats). The first two options are incorrect because the "second floor" of a building is specific information that will not take indefinite articles. So, the correct way to frame the sentence is, “My office is on the second floor, and I usually take the office elevator. But today, I will be using the stairs, and I usually take the office elevator. But today, I will be using the stairs.