Answer would be (D) It is variable.
TINYBLOB, TINYTEXT L + 1 bytes, where L < 2^8
BLOB, TEXT L + 2 bytes, where L < 2^16
MEDIUMBLOB, MEDIUMTEXT L + 3 bytes, where L < 2^24
LONGBLOB, LONGTEXT L + 4 bytes, where L < 2^32
Variable-length string types are stored using a length prefix plus data. The length prefix requires from one to four bytes depending on the data type, and the value of the prefix is L (the byte length of the string). For example, storage for a MEDIUMTEXT value requires L bytes to store the value plus three bytes to store the length of the value.
Here is the Reference.