In the first 2 hours, he completed 30% of the job
In the next 5 hours, he completed 70% of the job
Work = Rate x Time
Workdone in the first 2 hours = $$\frac{30}{100}\cdot210=63\ letters$$
$$Tom's\ rate\ in\ the\ first\ 2\ hours\ =\ \frac{63}{2}$$
$$Workdone\ in\ the\ next\ 5\ hours=\frac{70}{100}\cdot210=147\ letters$$
$$Tom's\ rate\ in\ the\ first\ 2\ hours\ =\ \frac{147}{2}$$
Rate of work for 2 hours : Rate of work for 5 hours
$$\frac{63}{2}:\frac{147}{5}$$
$$\frac{Rate\ for\ 2\ hours}{Rate\ for\ 5\ hours}=\frac{\frac{63}{2}}{\frac{147}{5}}=\frac{63}{2}\cdot\frac{5}{147}=\frac{315}{294}$$
$$=\frac{15}{14}\ \ \ \ \ \ \ \ \ \ Answer\ =\ option\ D$$