For high frequencies PCB track impedance is important parameter.
If you look at PCB trace impedance calculator like this one for instance you will notice that the impedance depend on trace width.
In Case A the PCB trace width change significant when the PCB turns on 90 degree in this spot PCB trace width is 1.4 times bigger, this creates low pass filter and such traces behave bad on high frequencies.
In Case B the angle is 45 degree and the trace width changes only 1.1 times (even less down to 1.05 times as plotters make rounding of the outer edge of the tracks), so this is acceptable in most of the cases. The traces with 45 degree also create less length of the traces than 90 degree traces ( Pythagoras theorem) so they are good to be used even with no so high frequencies as create less loop area to catch noise.
Case C is best as in this case PCB traces are with even width and have constant impedance. Most of the PCB routers though have no good routing with arcs and create them with thousands of small arcs (Eagle for example does this) and create enormously big files to plot, so not very practical to use it everywhere on the PCB. This is preferred way to route high speed signals like DDR memories or other high frequency signals.