What is the equivalent degree of MPhil in the American education system? I am applying to some universities for a Ph.D. degree, and I have to list all the degrees I have earned or am expected to earn. One of such degrees is Mphil. However, it is not listed in the drop-down menu of options.

There are three options that I think could fit--'Other Doctoral(not Phd)', 'Other Doctorate', and 'Other Master'. Which one would be the correct option? 'Other Master'? Since Mphil stands for master of philosophy?
Masters in the USA are (traditionally) taught.  The concept of a master in research/Mphil is an alien concept.  There might be some programmes but I'm not aware of any.

For your application purposes just say Other Master and explain in your application letter that it was a research degree.