Transistor Reverse Biasing - AC
You have a bunch of problems with your design.



Figure 1.


You are feeding 21 V into a 3.3 V micro-controller output. Unless it has high voltage open-collector outputs this won't end well.




Figure 2. The problem is similar to this High-side fail. The internal protection diodes will conduct and the LED will light continuously. The internal protection diodes may burn out and then the output stage will be over-stressed and fail.


Connecting Option A will feed a permanent 'high' into the base of the transistor. All the circuitry to the left then becomes irrelevant.



  The main issue I have encountered in past (Using only Opto-isolator to drive relay) is the output is accidentally connected to the 120V AC supply ...



You seem to have solved the 'accidentally" problem by permanently connecting it to the 120 V, 60 Hz supply! As shown you are feeding 120 V AC into your circuit, the transistor, the 100k resistor and possibly back into the supply. This is not correct and needs to be redesigned.
You are showing your 120 V supply referenced to earth. You should be showing it referenced to neutral. You can call this ground or GND and use the GND symbol (a triangle as on my Figure 2) and reserve the 'earth' symbol for mains earth.


Fixes


Power the opto-LED from the 3.3 V power rail. The forward-voltage, Vf is only 1.4 V so you have plenty of headroom.
A solid-state relay (SSR) is a readily available solution to replace the rest of the circuit and will save you much trouble. You need to select one that will work with a 3 V input and handle 120 V AC at the current you require. (Over-spec the current rating for safety margin.)
If you must roll your own circuit then replace your opto-isolator with an opto-triac. You will find plenty of examples in a web search.




From the comments:


  I want to switch 24 V relay, but to make the circuit fail-safe I want to add protection of some kind which can prevent damage to internal circuit due to 120 V supply connection. In other words -24 V rated output with, 120 V protection.


This is a very strange requirement. As you have drawn it the 120 V supply will probably have enough energy to destroy your protection devices and then destroy the rest of the circuit. We protect against this type of fault by choosing good quality, well designed components and wiring correctly.







simulate this circuit &ndash; Schematic created using CircuitLab

Figure 3. A relay or SSR adequately isolates the micro-controller from the mains.