Final answer:
The smaller strike zone introduced in 1969 led to improved batting statistics and worse pitching statistics due to the increased difficulty pitchers faced in throwing strikes. Therefore, the correct option is D.
Step-by-step explanation:
The strike zone in professional baseball was made smaller in 1969. The affect this had on the game was that batting statistics tended to improve because a smaller strike zone makes it harder for pitchers to get strikes, leading to more walks and better hitting conditions.
Consequently, this usually means that pitching statistics got worse, since pitchers are facing a tougher task in throwing strikes and getting batters out. So the correct answer to the effect of the smaller strike zone would be: D. Batting statistics stayed about the same, but pitching statistics got worse.