After the statements that follow are executed,
var firstName = "Ray", lastName = "Harris";
var fullName = lastName;
fullName += ", ";
fullName += firstName;

a. firstName="Harris"
b. firstName="Ray Harris"
c. fullName="Ray Harris"
d. fullName="Harris, Ray"

d. fullName="Harris, Ray"

Explanation:

// here we define two variables firstName="Ray" and lastName= "Harris"

var firstName = "Ray", lastName = "Harris";

// then we create another variable fullName and assign it the value stored in variabl lastName that is "Harris"

var fullName = lastName;

// then we added a coma and space. At this point fullName= Harris,

fullName += ", ";

// then we added firstName to the variable fullName. At this point fullName=Harris, Roy

fullName += firstName;

Note: the += operator means that add the variable on the left by an amount on the right

a+= b; or a = a + b; both mean same

answer; ///(d) subdatasheet;

answer; (a) brake pedal is depressed;