What is the power of the alphabet with which a message containing 1536 characters is written, if its volume is 0.75 Kbytes.

To understand what cardinality the alphabet has, it is necessary to determine how many bits / bytes each element is encoded. First, let’s determine how many bytes are in the source text:
0.75×1024 = 768 bytes (the calculation is made on the basis that there are 1024 bytes in one kilobyte).
Let’s determine the number of bits in the source text (there are 8 bits in one byte): 768 * 8 = 6144.
Divide the resulting number of bits by the number of characters in the text: 6144: 1566 = 4.
Thus, one character of the alphabet is encoded in 4 bits (0.5 bytes).
4 bits can encode 2 ^ 4 = 16 characters.
Answer: 16.