Answer
The beat frequency is $0.2~Hz$
Work Step by Step
We can find the frequency of the third harmonic of the cello string:
$f_3 = (3)(65.40~Hz) = 196.2~Hz$
The beat frequency is the difference in the frequency of the violin string and the third harmonic of the cello string. The beat frequency is $196.2~Hz - 196.0~Hz$ which is $0.2~Hz$
