Explanation:

A 'nibble' (sometimes spelled 'nybble') is a standard unit of digital information consisting of exactly four bits. It is equivalent to half of an eight-bit byte. The sequence "1001" contains precisely four binary digits. Option (b) Nibble is correct. Option (a) Byte is a group of eight bits. Option (c) Bit is a single binary digit. Option (d) Input refers to data entered into a system, not a specific unit of four bits. Therefore, 'Nibble' is the accurate term for a series of four bits, often used to represent a single hexadecimal digit.