The correct option is
B In
The concept of preposition is required to determine the correct answer.
Option
A- On is used to indicate place, specific date, direction, by means of. It does not fit correctly in the given sentence. Hence it is
incorrect.
Option B- For is used to indicate a space of time, use of something, to show who is intended to have or use something. It is
incorrect as it does not bring out the meaning of the sentence.
Option C- In is used to indicate inclusion
within
space,
a
place,
or
limits, inclusion
within
or
occurrence
during
a
period
or
limit
of
time. The given sentence indicates an occurrence with a given period of time. Hence 'in' is the correct answer.
Option D- At is used to indicate a point of time or place, state, direction. age or occupation, degree. Thus it is incorrect.