What is the best type of bicycle for adult beginners?
WHERE & HOW do you plan on riding? Answer that question first. Then get your butt over to a REAL bicycle shop. Why? Because you can TEST RIDE them before the sale. Plus friendly & knowledgeable sales help. Try that at Kmart, Target or Wally World, where all you'll find is junk bikes & clueless, minimum wage "associates". Basically, this SAME QUESTION was asked 2 weeks ago. Same answer. Here's a link to that question & answer... https://answers.yahoo.com/question/index?qid=20181210122718AAnJqhS
I think fit is the most important aspect of choosing a bike. A bike-shaped object from Wally World can likely be made to fit, but will not work out-of-the-box for most people. If you can't afford a bike shop bike, at least try to get one from a middle-retailer such as Dick's, who will have staff to help you get the correct size. Most beginners tend to want flat/riser bars, which means mountain bikes, comfort bikes, hybrids, and possibly cruisers.
What sort of adult beginner are you? Are you currently able to ride a bicycle? If not, the answer will not be the same as if you are able to ride, but only wanting to find a bike to improve your cycling ability Define "adult beginner" as it pertains to your question
Your question is very vague. Therefore the best answer I can give you is to go to a reputable bicycle shop. Tell them the type of riding you want to do and where you'll predominantly be riding the bike. They'll then make suggestions and fit you with the correct frame size for your body build. Upon purchase they'll adjust the bike to custom fit you for the greatest comfort.
Buy a hybrid bike. They are relatively easy to ride, have a more relaxed frame geometry and are suitable for road and gentle off-road riding.
A child's bike w/training wheels
Where do you plan on riding? I'd go with a hybrid too.
If it's a pedaling bicycle I would get a unisex mountain bike. They're low to the ground and easy to mount and dismount from.
For an adult who is just learning to ride a bicycle, I'd probably recommend they go with a mountain bike style w/ no more than 12 gears. These are cheap and readily available in any department store like Walmart. The fatter tires and ability to go off the pavement will be helpful to someone with no experience.
One that works