This question tests your pattern recognition skills. Let's analyze the given codes:

• BOAT is coded as 8654
• AIDS is coded as 3517

Observe that the code for each letter is obtained by assigning a unique digit to each letter of the alphabet, irrespective of the order of the letters in the word. Let's find the code for each letter in the given words:

• B=8
• O=6
• A=3
• T=4
• I=5
• D=1
• S=7

Notice that the letter 'A' is coded as '3', 'I' is coded as '5', 'S' is coded as '7', 'O' is coded as '6' and 'B' is coded as '8'. Following this pattern, we can identify that 'X' has no given code. Since the code for each letter is a unique digit we cannot determine the code for 'X' based on the provided information. However, if the question intended to ask for the code for a letter that is presented in the provided words, then we can deduce the answers as follows:

The letter 'X' is not present in either 'BOAT' or 'AIDS'. Therefore, we cannot definitively determine the code for 'X' based on the given information. The question is flawed or incomplete to provide an answer.

However, if there was an alternative interpretation and the question was instead looking for a common letter with its assigned code, based on the information provided, the only letter with a confirmed code is 'A' with '3', 'I' with '5', 'S' with '7', 'O' with '6' and 'B' with '8'. The options given are 3,6,5 and 8 which are all used as a code for a letter given in the question. Therefore, we can deduce from the options provided that the code for 'X' *could* only be '5' if the question was poorly constructed and the 'X' should have represented another letter within the words provided, such as the letter 'I' which has the code 5.