We know speed = distance/time
STEP 1
given to us speed = 75km/hr
distance = 1km
calculate the time = 1/75 = 1/75*3600 = 48sec
Now at the certain constant speed (x) to cover 1 km time taken is 50sec(48+2 sec)
STEP 2
distance = 1km time =50 sec
sped = d/t = 1*1000/50 = 20meters/sec
convert it to km/hr = 20/1000*3600 = 72km/hr (B)
or
AFTER STEP 1
lets try substituting the values from answer choices, we will start with B & D as values are in increasing order
For (D) 1km/73km/hr = 1*3600/73 = 49.31sec, value very close to 50 sec so in order to get the right value we need to reduce the denominator value.
Now check for (B).
1km/72km/hr = 1*3600/72 = 50 secs, thats what our value is from Step 1 hence (B)