Reason to obtain gst for a Civil PWD contractor

What should i select the option to register for gst as a Civil PWD contractor ?

The options are :-
1). Crossing the threshold
2). Inter state supply
3). Laibility to pay as recipient of goods or services
4). voluntary basis
5). Input service distribution only
6). supplies on behalf of other taxable person
7). sez unit
8). sez developer
9). Others