Answer:
D. 1916 and 1920
Explanation:
The following year spans, which had the greatest decrease in voter turnout rates is between 1916 and 1920.
From the above image, it is clear that there is a sharp and great drop in voter turnout between 1916 and 1920. It can seen that in 1916, the voter turnout is around 61.00% but in 1920, it dropped to around 48.00% (note: these my percentages are approximate. It's actually for the sake of explanation).
Therefore, 1916 and 1920 are the year spans that have decrease in voter turnout.
