We encourage tinkering with problems to make them yours! Please just make a copy of an existing problem so it will be in your personal bank (and feel free to share it in other banks that you see relevant too).
1) use a numeric free response question (which is coded in TeX) as template. You'll know because there will be just a question, with no choices provided in the upper TeX box question part (the expl is the explanation). 
2) To convert a TeX open to TeX multiple choice three additions are needed:
in the upper TeX box, we have to put in choices:
\choice{}{1}{@t}
\choice{}{2}{@dist1}
\choice{}{3}{@dist2}
\choice{}{4}{@dist3}
\choice{}{5}{@dist4}
....can go up to 10! everything except what's in bold needs to be there.
In the lower code box, we have to define the variables and describe what those distractor answers should be
3a) define the variables in the code box
/* global double dist1 u={units} */
/* global double dist2 u={} */
/* global double dist3 u={} */
/* global double dist4 u={} */
This says that dist1-4 will be recognized in the lower code box and upper TeX box; hence global double. The specific units may be placed as indicated above.
3b) Now we direct what we want the distractors to be:
dist1 = omega*pi/60.0/alpha;
dist2 = omega*2.0*pi/alpha;
dist3 = 2.0*pi/60.0/alpha;
dist4 = omega*2.0*pi/60.0;
use variables already defined.
5) The answer itself also needs one tweak–unlike in NFR where the actual answer or variable is identified as ans1 (ex: ans1=@a1), for multiple choice define ans1 as the answer choice that has the correct variable listed as the solution among the multiple choice boxes (ex: ans1=1 , where \choice{}{1}{@a1}).
t = omega*2.0*pi/60.0/alpha;
ans1 = 1;
ans1 =1 means the first answer choice in the upper code box is the correct one.
In multiple choice the ans1= in the code box has to be the choice with the correct answer (ie, ans1 = 1; instead of ans1 = a1;), not what's being called.
If you're getting 'tex validation errors' please check that ans1 = a choice number instead of a variable.
Pro tip: just make a precursor ans = expression on how to calculate the answer. In the upper TeX box, in choice 1 put in ans, and in the bottom Code box, ans1=1.
6) check the upper TeX box explanation to ensure that the final answer displayed will be (in this case, t)
@ means the variable is being summoned from the code box to the TeX box–the latter which is what is displayed.
Generate parameters, and if there are no errors, proceed to publish! Feel free to put in relevant all category banks so others can use your well crafted question too.