Within the standard MCQ question type in Core Moodle where questions generally have only 1 correct answer there is an option to allow multiple answers (check all that apply).
If this option is selected each answer carries it’s own grade % of the question so if A and B are both correct, the configuration would be that A and B carry 50% each and that C and D are worth 0 (zero). A student answering only A or only B would receive partial credit.
The All or Nothing MC Qtype fixes that issue,
The all-or-nothing multiple choice question is a version of the core multi-answer multiple choice question with modified grading. In an all-or-nothing multiple choice question, a respondent can choose one or more answers. If the chosen answers correspond exactly to the correct choices defined in the question, the respondent gets 100%. If he/she chooses any incorrect choices or does not select all of the correct choices, the grade is 0%.
If this is something you’d like your site to have you can download it here: https://moodle.org/plugins/view/qtype_multichoiceset
All-or-Nothing Multiple Choice is available for Moodle 2.6 and up (including 2.9).