Threading refers to the grooves around the inside of the nut and on the outside of the bolt. There are two basic thread classifications. Coarse threads have a smaller number of "turns" or grooves and fine threads have a greater number. Coarse threaded bolts require coarse threaded nuts. Fine threaded bolts require fine threaded nuts in order to be compatible. Metal alloys have nothing to do with thread specifications. Answer choice "4" defies common sense, as does choice "2."