The LCM of 2, 3, 4, 5, and 6 is 60.
So, rewrite all fraction with denominator 60.

$\frac{1}{2}+\frac{2}{3}+\frac{3}{4}+\frac{4}{5}+\frac{5}{6}=\frac{30}{60}+\frac{40}{60}+\frac{45}{60}+\frac{48}{60}+\frac{50}{60}$

$=\frac{30+40+45+48+50}{60}$

$=\frac{213}{60}$

NOTE: For GRE questions that require you to enter a fraction, all equivalent fractions are correct. So, for example, if the answer is 1/2, you'll get the question right if you enter 3/6, 5/10, 1/2, 222/444, etc

So, we need not risk incorrectly simplifying $\frac{213}{60}$

Answer: $\frac{213}{60}$ or $\frac{71}{20}$ or $\frac{2130}{600}$ or ....

Cheers,
Brent