When a Multiple Choice question is scored, any student who selected ALL the correct responses and ONLY the correct responses will get full credit, and anyone else will get 0 points. If you want the scoring to be more flexible, you can use the "Partial Credit" setting which means that students will be awarded a percentage of the points for the question based on their overall accuracy.
A student's accuracy for a question is equal to the correct selections they made divided by the total selections they made (or the total number of correct responses, if they made fewer selections than there are correct options).
An example -
A question with two correct responses and three distractors
Student A selects the two correct responses and nothing else: 2 correct out of 2 selections is 2/2 for 1, for 100% credit
Student B selects one correct response and one distractor: 1 correct out of 2 selections is 1/2 or .5, for 50% credit
Student C selects one correct response and nothing else: 1 correct out of 2 potential correct is 1/2 or .5, for 50% credit
Student D selects all 5 responses: 2 correct out of 5 selections is 2/5 for .4, or 40% credit
Partial distractor credit
If you are using the "Partial Credit" setting for a question, you also have the option to give partial distractor credit. This feature lets you award some credit to a student for selecting a certain distractor, in cases where the distractor may be close to correct or otherwise worthy of earning some credit. To designate a distractor as worthy of credit, you just give it a percentage value in the "Partial credit" field - the higher the percent, the more points the selection will earn.
Each student's partial credit is still based directly on their overall accuracy for the question, and the calculation is the same - correct selections divided by total selections. However, after adding up the fully correct selections, each partial credit selection is counted as well - each adding its designated percent to the total.
An example:
We will add a sixth option to our question above, this one a distractor worth 50% partial credit.
Student A selects the two correct responses and nothing else: 2 correct out of 2 selections is 2/2 or 1, for 100% credit
Student B selects one correct response and the partial credit distractor: 1.5 correct out of 2 selections is 1.5/2 or .75, for 75% credit
Student C selects the partial credit distractor and one distractor: .5 correct out of 2 selections is .5/2 for .25, or 25% credit
Student D selects 5 responses, everything but one incorrect distractor: 2.5 correct out of 5 selections is 2.5/5 or .5, for 50% credit