In an aptitude test, there are $20$multiple choice questions, each question having four options (a, b, c, d). If all the options are equally likely, then what is the most probable score of Shyam who has marked option 'a' for all the questions? Assume that a student is penalized with $\frac{1}{2}$ a mark for each wrong answer and $+2$ is awarded for each right answer.