saurya_s wrote:

A researcher plans to identify each participant in a certain medical experiment with a code consisting of either a single letter or a pair of distinct letters written in alphabetical order. What is the least number of letters that can be used if there are 12 participants, and each participant is to receive a different code?

A. 4

B. 5

C. 6

D. 7

E. 8

Ans = B (5)

Use A, B, C,D& E as the 5 codes.

5 participants can each get one of the 5 codes.

For the remaining, use 2 code combinations from the 5 codes

10 such combinations are possible.

Hence 5 codes are suff.