jQuery conditional statement to check if a select option has either of 4 values selected
// You can use the document.ready as both encapsulation as well as
// to make sure the DOM is ready for operation.
$(function ($) {

  // Unless these are dynamic (the set could change), it's better to cache them
  var optionBox = $('.optionBox');
  var dropOff = $('#dropOff');
  var pickUp = $('#pickUp');
  var field1 = $('#field_1');

  field.show();

  // Your select box has an ID. They are faster than attribute selectors 
  // in both parsing ase well as fetching from the DOM
  $('#serviceChoice').change(function () {

    // The current context of this function is the DOM element, which is the select
    // You can use it to get the value, rather than using a selector again. Removes
    // the need for jQuery to parse the selector and look for it in the DOM
    switch ($(this).val()) {
    case 'A':
      optionBox.hide();
      dropOff.show();
      break;
    case 'B':
    case 'C':
    case 'D':
      optionBox.hide();
      pickUp.show();
      break;
    default:
      optionBox.show();
    }
  });
});


Minus the comments, here's what you get:

$(function ($) {

  var optionBox = $('.optionBox');
  var dropOff = $('#dropOff');
  var pickUp = $('#pickUp');
  var field1 = $('#field_1');

  field.show();

  $('#serviceChoice').change(function () {

    switch ($(this).val()) {
    case 'A':
      optionBox.hide();
      dropOff.show();
      break;
    case 'B':
    case 'C':
    case 'D':
      optionBox.hide();
      pickUp.show();
      break;
    default:
      optionBox.show()
    }
  })
});