For the circuit shown in the figure above, the correct procedure for measuring the resistance value of r1 is to

a. connect the multimeter between points d and a and measure the resistance with switch s1 open.
b. connect the multimeter between points c and d and measure the resistance with switch s1 open.
c. connect the multimeter between points c and d and measure the resistance with switch s1 closed.
d. connect the multimeter between points d and a and measure the resistance with switch s1 closed.

B. connect the multimeter between points C and D and measure the resistance with switch S1 open.

Explanation:

multimeter is a device which is used to measure the resistance of wire across which it is connected.

Here we need to connect the two ends of the multimeter across the two ends of resistance and then it's reading is to be measured.

Here we need to take care that while measuring the resistance the circuit resistance current must not flow through it.

So here we need to make sure that switch S1 must be open while measuring the resistance

so correct answer will be

B. connect the multimeter between points C and D and measure the resistance with switch S1 open.

it is easily magnetized.

frequency or wavelenth of em radiation in a vacuum