Got-it shorthand (ryo)
Overview
Ryo is a reply made by shortening ryoukai, which means "Got it." It goes from ryoukai to ryo, and sometimes gets cut down even further to just ri. It's youth slang used on LINE (a messaging app widely used in Japan) and in other chats.
This kind of shortening has the upside of making text communication more efficient. The meaning gets across, and you have fewer characters to type. But the person receiving it may feel it's "sloppy," "cold," or "not interested." Shortened words come with a trade-off between efficiency and warmth.
Is ryo okay in a question box?
Answering with nothing but ryo in a question box could disappoint the sender. Some senders work up the courage to send a question. For them, a reply of just 2 characters becomes a reason to feel "so that's all my question was worth."
But depending on the context, ryo can work well. One example is replying ryo on purpose in a light exchange between friends to show you're close. "Let's hang out sometime." "Ryo." Here, ryo isn't cold. It's a way of being casual. The same 2 characters can mean different things depending on the relationship and the context.
The art of the short reply
Longer isn't automatically better for question box answers. Some answers are short but heartfelt. The problem with replying only ryo is less about how short it is and more that your respect for the sender is hard to get across.
An example of a good short answer: "Good question. The answer is X. The reason is Y." This takes only 3 lines, but it values the question, gives an answer, and adds a reason. What matters is density, not length.
On the other hand, some answers are bad even though they're long. One example is an answer where an empty lead-in goes on and on and the conclusion gets blurry. The opposite of ryo isn't "a long answer" but "a dense answer."