“Heat cannot by itself flow from a body at lower temperature to a body at higher temperature” is a statement or consequence of

Entropy of a system decreases .........

Why can't the efficiency of a heat engine ever be $100 \%$?

One mole of an ideal gas at $300 \ K$ in thermal contact with surroundings expands isothermally from $1.0 \ L$ to $2.0 \ L$ against a constant external pressure of $3.0 \ atm$. In this process,the change in entropy of the surroundings $(\Delta S_{\text{surr}})$ in $J \ K^{-1}$ is $(1 \ L \ atm = 101.3 \ J)$.

"Heat cannot flow by itself from a body at a lower temperature to a body at a higher temperature". This statement corresponds to:

$Assertion :$ When a glass of hot milk is placed in a room and allowed to cool,its entropy decreases.
$Reason :$ Allowing a hot object to cool does not violate the second law of thermodynamics.