1. Identify the core configuration

Einsteinium has atomic number $Z=99$.

The noble gas core just before the actinide series is radon, $[\mathrm{Rn}]$, which accounts for $86$ electrons.

So, electrons beyond radon: $99-86=13$

2. Actinide filling pattern

In the actinide series, electrons are filled mainly into the $5f$ subshell, with $7s$ already having $2$ electrons.

So after $[\mathrm{Rn}]$, the general pattern is: $[\mathrm{Rn}]\,5f^n 6d^0 7s^2$

Since $7s^2$ uses $2$ electrons, the remaining electrons in $5f$ are: $13-2=11$

Thus the configuration becomes: $[\mathrm{Rn}]\,5f^{11}6d^0 7s^2$

3. Match with the options

• A: $[\mathrm{Rn}]\,5f^{13}6d^0 7s^2$
  Total beyond $[\mathrm{Rn}] = 13+2=15$ electrons $\Rightarrow Z=101$ ❌

• B: $[\mathrm{Rn}]\,5f^{12}6d^0 7s^2$
  Total beyond $[\mathrm{Rn}] = 12+2=14$ electrons $\Rightarrow Z=100$ ❌

• C: $[\mathrm{Rn}]\,5f^{11}6d^0 7s^2$
  Total beyond $[\mathrm{Rn}] = 11+2=13$ electrons $\Rightarrow Z=99$ ✅

• D: $[\mathrm{Rn}]\,5f^{10}6d^0 7s^2$
  Total beyond $[\mathrm{Rn}] = 10+2=12$ electrons $\Rightarrow Z=98$ ❌

4. Final answer

The correct electronic configuration of Einsteinium is: $[\mathrm{Rn}]\,5f^{11}6d^0 7s^2$

So, the correct option is C.