When cart moves with some acceleration towards right then a pseudo force (ma) acts on block towards left
This force (mα) is action force by a block on cart. Now, block will remain static with respect to the cart, if frictional force f=mg ∵‌‌f≤µR⇒µR≥mg ⇒‌‌µmα≥mg[R=mα] α≥‌