[Old Pedant]

Lookup table is easiest. External file may be best. You have to decide that part.

Lookup table is trivial:

var phrases = [
     null, /* 0 is never used */
     "this is phrase 1",
     "this is phrase 2",
     ..
     "this is phrase 140"
];
var chosenPhrase = phrases[document.getElementById('datamem14').value];
if ( chosenPhrase == null ) chosenPhrase = "That is not a valid selection";

As an alternative, if you wanted the value from datamem14 to be something other than a number form 1 to 140:

var phrases = {
    "auto" : "This is automatic",
    "manual" : "5 speed on the floor",
    "chain" : "Must be a bicycle",
    ...
}
var chosenPhrase = phrases[document.getElementById('datamem14').value];
if ( chosenPhrase == null ) chosenPhrase = "That is not a valid selection";