A. `timescale 1ns/1ps; reg clk =0; always #50 clk = ~clk

B. `timescale 1ns/1ps; reg clk =0; always #5 clk = ~clk

C. `timescale 1ns/1ps; reg clk =0; always #0.5 clk = ~clk

D. `timescale 1ns/1ps; reg clk =0; always #500 clk = ~clk