A Dungeons & Dragons group that is short on players should select specific character classes for small parties, in order to increase their chances of survival. The most difficult task facing any D&D party isn't killing a Tarrasque or casting an epic level spell: it's aligning the schedules of several adults at once.
The standard four-character D&D party layout has been set in stone for a long time. Right in front, there is the tank (Barbarian, Fighter, Paladin), who soaks up damage and deals heavy blows of their own, while keeping the squishier members of the party safe. Next, there is the healer (Bard, Cleric, Druid) who keeps everyone alive with their buffs and spells. The third character is the Rogue, who spots the traps and deals powerful attacks from the enemy's blind spots. Finally, there is the caster (Sorcerer, Warlock, Wizard), who bends the rules of the universe to their will, in order to tip the odds in the party's favor.
It's not always possible to gather a full group of players together for D&D, and not all players are comfortable running two characters. The group might also not want to use D&D Sidekicks, even though they can be helpful in bolstering the group's numbers. If the small party is going to survive its dungeon crawl, then the players will have a better chance of staying alive if they are careful with their choice of character class.
There are times when a single adventurer must venture out into the world. There are also times when DMs will run a solo game for a player in order to give them a feel for the rules. In these cases, it's important that a character can survive long enough on their own. The best way to do this is with a character that can deal damage, has high AC, and the ability to heal themselves. The hard-to-kill D&D Paladin is the best option for this, as they can wear heavy armor, can use shields, and can utilize Lay On Hands to heal their wounds. This is bolstered when they hit level two, when they gain the ability to cast Cure Wounds. The Fighter is a good second choice, as they can use Second Wind for some extra healing. If the character starts at level two, then the Ranger is also a good choice, as that's the level they gain access to healing spells.
Once a second character is added to the mix, the best option is the Rogue. The expansive skill set of the Rogue is necessary for surviving many encounters, in terms of both dungeoneering and dealing with NPCs. Sneak Attack also deals a ton of damage when coupled with another character, and their Cunning Action ability helps them to stay alive for longer.
Once a third character enters the mix, they should bring some arcane might to the group. A Sorcerer or a Wizard would be a good choice, as they have far more utility than the Warlock. Having some extra party members around means that the arcade caster has a buffer so that they can cast their spells without fear of being shanked. The spellcaster brings a ton of options to the table for dealing with challenges, in terms of destroying enemies, deceiving foes, or bypassing hazards. Once a fourth character comes to unify the party, they should be a Bard/Cleric/Druid, but at that point, they'll have brought the numbers up and created a standard Dungeons & Dragons party.
