What is the highest frequency contained in a pulse that has a rise and fall time equal to 10 microseconds (10 s)?

ANS:A - 35 kHz

Rise Time/Fall Time. Rise time refers to the time it takes for the leading edge of a pulse (voltage or current) to rise from its minimum to its maximum value. Rise time is typically measured from 10% to 90% of the value.

So the extra time required to get 10% value can be any value.

If we consider it as say 5 us then also we get 40kHz so, this might be the reason that, maximum frequency form given options will be 35kHz.