If ROSE is coded as 6821, CHAIR is coded as 73456 and PREACH is coded as…

This is a letter-to-digit substitution cipher. Each given code word fixes the digit for every letter it contains; when the same letter reappears across different code words, it must always map to the same digit. Pooling these letter-digit pairs from all the given words builds a complete key, which is then applied letter by letter to encode the target word.

1. From ROSE = 6 8 2 1: R = 6, O = 8, S = 2, E = 1.

2. From CHAIR = 7 3 4 5 6: C = 7, H = 3, A = 4, I = 5, R = 6 (R repeats from ROSE and matches).

3. From PREACH = 9 6 1 4 7 3: P = 9, R = 6, E = 1, A = 4, C = 7, H = 3 (R, E, A, C, H all repeat from the earlier words and match).

4. Pooling all three words gives the complete key: R = 6, O = 8, S = 2, E = 1, C = 7, H = 3, A = 4, I = 5, P = 9.

5. Apply this key letter by letter to SEARCH (S-E-A-R-C-H): S = 2, E = 1, A = 4, R = 6, C = 7, H = 3.

6. Writing these digits in order gives the code 214673.

Every repeated letter -- R, E, A, C, H -- maps to the identical digit in every word it appears in, confirming the key is consistent before it is applied to SEARCH.

Hence, the code for SEARCH is 214673.