The text takes 0.25 KB of computer memory. How many characters does this text contain?

Everything very much depends on the encoding. It is there that indicates how many bits will be allocated in order to display any character.
Since the problem does not indicate which encoding the text belongs to, we will assume that it is standard and is characterized by 8 bits for one character or one byte.
The size of our text is 0.25 kb, which means that when converting this size into bytes, we will need to multiply this value by 1024:
0.25 kb = (0.25 * 1024) b = 256 b.
Since there is one byte per character, and there are only 256 bytes, there will be the same number of characters.
ANSWER: 256 characters.