doubtyyyyyyyyy

The force which is sufficient to just let it move down:
here F will act up the plane
mgsinα down the plane
friction = μmgcosα up the plane
So, F = mgsinα - μmgcosα

The force which is sufficient to just let it move up:
here F' will act up the plane
mgsinα down the plane
friction = μmgcosα down the plane
So, F' = mgsinα + μmgcosα

it is given F' = nF
So, (sinα + μcosα) = n(sinα - μcosα)
So, μ = sinα(n-1)/cosα(1+n) = option (a)