A test consists of 6 multiple choice questions, each having 4 alternative answer of which only one is correct. The number of ways, in which a candidate answers all six questions such that exactly four of the answers are correct, is _____

we can select 4 questions out of 6 questions in C(6,4) ways and  select the correct  options in only 1 way.Now we can select wrong options for the remainng 2 questions in 3×3 ways.number of ways =C(6,4)×1×3×3                                  =135