The atomic number of Unnilunium is______.

To determine the atomic number of the element whose temporary IUPAC name is “Unnilunium”, we recall the systematic naming convention introduced by the International Union of Pure and Applied Chemistry (IUPAC) for elements whose permanent names were not yet assigned.

According to this convention, the name of an element is built from three-letter Latin/Greek numerical prefixes that denote each digit of its atomic number, read from left to right, and finished with the ending “-ium”. The officially adopted prefixes are:

$$\text{un} = 1, \qquad \text{bi} = 2, \qquad \text{tri} = 3,$$

$$\text{quad} = 4, \qquad \text{pent} = 5, \qquad \text{hex} = 6,$$

$$\text{sept} = 7, \qquad \text{oct} = 8, \qquad \text{enn} = 9,$$

$$\text{nil} = 0.$$

In the given name “Unnilunium” we can clearly identify three numerical prefixes followed by the suffix “-ium”. We write them one after another:

$$\text{un} \;+\; \text{nil} \;+\; \text{un} \;+\; \text{ium}$$

Now we translate each prefix into the corresponding digit:

$$\text{un} \rightarrow 1, \qquad \text{nil} \rightarrow 0, \qquad \text{un} \rightarrow 1.$$

Placing these digits in the same order, we obtain the three-digit sequence

$$101.$$

So, the temporary systematic name “Unnilunium” represents the element whose atomic number is $$101$$. Historically, once the element received its permanent name, it became known as Mendelevium (symbol $$\text{Md}$$).

Hence, the correct answer is Option C.