Mutal Funds quite often use a $60/40$ balanced portfolio as a blend between equities and bonds. Please mark the only INCORRECT statement about the $60/40$ portfolio:

a$.$

The $60/40$ is not atractive when real bond yields are negative and bonds and equities are positively correlated

b$.$

The $60/40$ works because historically stocks and bonds have been negatively correlated

c$.$

Historically, the $60/40$ offers a sweet spot for asset allocation because its Sharpe ratio tends to be high

d$.$

A mutual fund manager can flip at any time the allocation to $40\%$ equities and $60\%$ bonds