What is the maximum decimal value of a nybble?

Are Memory addresses are most often displayed in the hexadecimal numbering system?

Yes. We could use decimal notation but hexadecimal is more convenient because it requires fewer digits and more closely reflects the way the machine addresses memory using its native binary notation. For instance, a 64-bit address in decimal requires 20 decimal digits (including leading zeroes) but only 16 hexadecimal digits. Moreover, the hexadecimal value can be easily translated into the actual binary value used by the machine because each hex digit maps 1:1 with every nybble of the binary value. A nybble is half-a-byte (4-bits). Since each address typically represents an 8-bit byte, the value of that byte can also be expressed using just 2 hexadecimal digits (00 to FF) whereas decimal notation would require 3 digits (000 to 255). If we used decimal notation to present the contents of a block of memory, then we wouldn't be able to fit as many columns of data on the screen at once. More importantly, when we look at the contents of memory we're generally more interested in what the computer sees, and hexadecimal notation more closely reflects what the computer sees.