The Question :
1. What is an RPG ?
An RPG is a game that allows you to take on a role and play within it, in its most simple terms, there are RPGs that don't require the same exp and levelling experience that we typically associate with an RPG, however most use this method to track character growth. The Artifact, and Incest Adventure are both RPGs but approach them differently. The artifact is what's known as an action rpg, giving you the player an active role in the open combat, but also showing a progression from the beginning to the end. Incest Adventure is a Turn Based RPG, this is the more traditional format that we are used to in games like Pokemon, or Final Fantasy.
2. What is a Sand Box ?
A Sandbox is technically just another word for an Open World game. However one could make the argument that the nature of a sandbox is that its a world in which you, the player, get to decide what to do and when to do it. Not all Open Worlds manage this well.
3. What is an Open World games ?
An Open World game is a game that allows you, the player, to explore it's entirety at your own pace, letting you go back to where you came from, or forward to a place you're not really meant to be yet at any time. Some of these games do this while keeping you locked in to a linear story, others allow you to move through it's story at your own pace. Big Brother, Man of the House, View of Family, etc, are all examples of open worlds, in that you, the player have a choice in where to go at any point. Similarly most of the RPGM titles are the same, allowing the MC to go and do what they want at any given time. However I wouldn't really call any of them sandboxes, because they all have linear progression and there's not any real fun to be had outside those systems.
4. What is a Visual Novel ?
Due to the fact that this place has a lot of games that fit this description it's hard to nail it down for certain. For instance, I would tell you that My Girlfriends Amnesia is a Visual Novel, however it uses the RPGM format. That said, it doesn't really use that format because it needs it, and could arguably be a better game if it was in Ren'Py. In essence a Visual Novel is a game wherin the interactions of the player are mostly limited to making choices, to do or not to do. It's a mostly linear storytelling medium, that, even with branches, can often be very much in a box designed to move to one definitive end.
5. Are all Games in F95 (exclude the action genre), are Visual Novel ?
Absolutely not. Some may feel like it, other's may use an engine that was intended for them, but I wouldn't call all the games here Visual Novels.
6. If no. 5, there RPG, Sand Box & Open World Games style, can you make 3 example ?
RPG I already mentioned Incest Adventure and The Artifact, I could add Icstor to the list to make an even 3
Sand Box I don't think we are there yet, however if anyone is interested and owns or can pirate skyrim, the mods OSA engine, Osex, and Amorous Adventures turn the game into a pretty interesting open world sandbox sex game. Have fun. The mods OSA engine and Osex also work in Enderal.
Open World My favorite would be Coceter Chronicles, which allows you to travel the world at your own pace, interacting with plenty of side quests that have no bearing on the main story. This might be the closest we have to a Sandbox, but I still wouldn't put it up there. Then obviously you have the Life sims, like View of Family and Chloe 18 that allow you to go around and do things at your own pace. There's no requirement that you go to any one place, and while certain places are closed during certain hours, that's no different from the real world.