If you ever get confused regarding what is what, look at the units and verify.
capacity C is equal to 100,000 bits per second.
This is the Bandwidth.
transmission rate (in bits per seconds) ...
They're asking for Sender Throughput.
Throughput = efficiency * bandwidth.
Efficiency in Stop-And-Wait = $\frac{1}{1+2a}$
$a = \frac{T_p}{T_t}$
=> $\frac{1s}{100bits*seconds/100000bits}$ (Notice the units cancel each other perfectly. This indicates we're going right)
=> $a=1000$
So, $\frac{1}{1+2a}$
= $\frac{1}{2001}$
=> efficiency = $ 0.00049975012 $
Finally, Throughput = $ 0.00049975012 * 100000bps=49.975 bps$