I'm going to assume you meant "rightmost" since the leftmost component is a supply.
The voltage across the rightmost component must be the same as any components in parallel. Hence, it has 5V across it.
If the dependent voltage source was a resistor there would be no current through it since one end of it is connected to an open circuit (at node a). A resistor with no current through it has no voltage through it (since \$V = IR\$), so a resistor would not affect \$V_{\text{TH}}\$.
But the dependent voltage source has a non-zero voltage because its value is \$30\times 10^3 i_0\$ (where \$i_0\$ is the current through the upper resistor), and \$i_0\$ is non-zero.
There is still no current through the dependent source because it is connected to an open circuit at node a. That means the current through both resistors (\$i_0\$) is the same: $$i_0 = -\frac{100\text{V}}{20\text{k}\Omega + 80\text{k}\Omega} = -1\text{mA}$$
\$i_0\$ is negative based on the direction indicated in the circuit. The dependent voltage source's voltage is therefore $$V_D = 30\times 10^3 i_0 = -30\text{V}$$
This needs to be added to the voltage at the node between the resistors (which is a simple voltage divider) to calculate \$V_{\text{TH}}\$.
Best Answer
The answer is, it is, and it isn't.You have missed one critical piece of information in your question, when. At steady-state, or during the transient period when the voltage is first applied.It seems you added the information in the comments. It is fully charged, i.e. steady-state condition.
In the transient the capacitor will charge up through the resistors until it reaches \$1\mathrm{V}\$. Once the capacitor has reached this voltage (i.e. it is fully charged), assuming it is ideal and the voltage source remains constant, then you will have:
$$V_s=V_c$$
Clearly that means all the voltage is dropped across the capacitor, so there cannot be any voltage across the resistors.
For completeness, we can look at the steady state condition in another way. The reactance of a capacitor (similar to resistance, but frequency dependent), is given by:
$$X_c = \frac{1}{2\pi fC}$$
Where \$f\$ is the frequency, and \$C\$ is the capacitance. At DC, the frequency is \$0\mathrm{Hz}\$, so the reactance is:
$$X_c = \frac{1}{2\pi C\times 0} = \frac{1}{0} = ∞$$
So what will the current be if the reactance is infinite? \$I=\frac{V}{X_c}=0\$. If there is no current flowing, there can be no voltage across the resistors \$V=IR=0\times R=0\$.