To convert the given plain text "Exam is today" into a cipher text using the mono-alphabetic substitution, we need a key that specifies the substitution mapping for each letter. In this case, let's use the key "SELFLESSNESS" as provided.
To create the cipher text, we will substitute each letter in the plain text with the corresponding letter from the key. Here's how it can be done:
Plain Text: E x a m i s t o d a y
Key: S E L F L E S S N E S S
Cipher Text: S E L F S S E S N E S S
Therefore, the cipher text for the given plain text "Exam is today" using the mono-alphabetic substitution and the key "SELFLESSNESS" would be "SELF SNESSNESS".
Note that in mono-alphabetic substitution, each letter in the plain text is replaced with a single corresponding letter from the key.