Selectable

static int selected = nk_true; /* outside main cycle */

...

/* within main cycle */
if (nk_begin(ctx, "Test", nk_rect(32, 32, 192, 192), NK_WINDOW_BORDER | NK_WINDOW_MOVABLE | NK_WINDOW_CLOSABLE))
{
    nk_layout_row_dynamic(ctx, 30, 1);
    nk_selectable_label(ctx, "label_1", NK_TEXT_LEFT, &selected);
}
nk_end(ctx);