In a certain code language, ‘bright colour rainbow’ is coded as ‘mq bj st’ and ‘sunny bright day’ is coded as ‘nv bj fm’. How is ‘bright’ coded in that language?

Correct — D. Compare the two sentences and the two code strings for what they share.

'bright colour rainbow' → 'mq bj st'
'sunny bright day' → 'nv bj fm'

The word appearing in both sentences is bright. The code appearing in both strings is bj.

A shared word and a shared code have to correspond, so bright = bj → option (d).

Why the others are wrong

• (a)
  st — st appears in the first code string alone, so it must stand for colour or rainbow — and neither word is in the second sentence.
• (b)
  mq — mq likewise belongs to the first string only, so it also codes colour or rainbow. The question gives no way to decide which, and no reason to attach it to bright.
• (c)
  nv — nv appears in the second string only, so it codes sunny or day, not the word the two sentences have in common.

Word-substitution coding is solved by intersection, never by decoding. You do not need to know what colour or sunny map to.

The two sentences share exactly one word, and their code strings share exactly one code. That single overlap is the whole solution.

Had the sentences shared more than one word, the intersection would hand you a set of possibilities and no way to separate them. Here the overlap is a single word, so the code is fixed.

Key facts

• 'bright colour rainbow' is coded 'mq bj st', and 'sunny bright day' is coded 'nv bj fm'.
• bright is the only word common to the two sentences, and bj is the only code common to the two code strings.
• The intersection method needs no assumption about the order in which the codes are printed.

Common traps

• Matching by position, pairing the first word with the first code
• Trying to decode every word rather than taking the intersection
• Choosing a code that appears in only one of the two strings

No directly related past PYQ was found.