Which of the following introduces an error of about 1 in 1000 if 20 m chain is used

ANS:E - all the above.

Slope Correction or error = h^2/2L.

Now we want to find the difference in height of chain ends for which 1 in 1000 or 20 mm in 20 m chain length will produce.

0.02 = h^2/(2 x 20),
h = 0.9 m,
So, option B along with A, C options is correct. Slope error= 20mm = h^2/2L.
Then, h=0.89m~0.9m.
i.e. one end is 0.9m higher/lower than another end. 1 in 1000 means 1000 m = 1m.
then 20 m = 1/1000* 20=20mm.