In a certain code, 'it is raining' is coded as 'dy po ko', 'is raining now' is coded as 'po jo ko' and 'is now it' is coded as 'po jo dy'. (All the codes are two-letter codes only.) What is the code for 'raining'?

Correct — A.

Rule: a word shared by two sentences must be coded by a code shared by those two sentences.

'raining' is in sentence 1 (dy po ko) and sentence 2 (po jo ko). Their shared codes are po and ko.
Those two sentences also share 'is', so one of po and ko is 'is' and the other is 'raining'.

'is' is in all three sentences, and po is the code that appears in all three, so is = po.
That leaves raining = ko → option (a).

Why the others are wrong

• (b)
  dy — dy appears in sentences 1 and 3, whose shared words are 'it' and 'is'. Since is = po, dy has to code it.
• (c)
  jo — jo appears only in sentences 2 and 3. Those two share 'is' and 'now', so jo codes now — and sentence 3 does not contain 'raining' at all.
• (d)
  po — po sits in all three code strings, but 'raining' is missing from the third sentence. The word present everywhere is 'is', so po = is.

Substitution coding hands you sentences, not words. You never decode letter by letter — you intersect.

A word present in two of the given sentences must map to a code present in both of their code strings. A word present in all three must map to the code present in all three.

Work the most constrained word first. Here that is 'is', which appears everywhere and fixes po; every later deduction rides on that one anchor.

Key facts

• 'is' occurs in all three sentences and po occurs in all three code strings, so is = po.
• 'raining' occurs in sentences 1 and 2, whose code strings share only po and ko.
• The full key here is it = dy, is = po, raining = ko, now = jo.

Common traps

• Lining the words up with the codes in the order printed — 'it is raining' and 'dy po ko' are not in matching order
• Stopping at 'po or ko' without using the third sentence to knock out po

No directly related past PYQ was found.