Well technically all of this except d) is true and a) is the most correct. The easiest explanation for me is following.
Sometimes you take the correct decision in certain situation but you loose. Other times you take the wrong decision but you win. Both cases are example of variance. Variance is the element of luck in the
poker game. It can help you sometimes and it can bring you down in other cases.
In fact variance is the thing that makes this game so interesting because the end result depends partially from your skill and partially from luck. If everything depends only on player's skill it would be chess and not poker and every time good player will win over weak player.
Variance can be very harmful sometimes and can cause serious loosing streaks where you play the game well, take the correct decisions and despite of that you continue to loose. Very important in such situations is to not allow this to tilt you influencing your game and to have very good
bankroll management and deep enough bankroll so that you can overcome such loosing streaks. Variance can also bring you winning streaks where everything is on your side and you win much more money than usual. This of course is something good but strict bankroll management is also very important here because many players are tempted in such cases to play at much higher stakes than they normally do and this is recipe for disaster.
In general if you play good, take the right decisions and have good bankroll management you should be able to mitigate the effects of variance and when you play more and more
hands variance will affect you less and less.