The correct option is
A To
Option B- To is used to indicate direction, purpose, limit, attachment,
comparing. Thus 'to' is the
correct answer.
Option A- At is used to indicate a point of time or place, state, direction. age or occupation, degree. Thus it is incorrect.
Option C- Of is used to indicate possession, relation, substance, cause
or about. As it does not fit the meaning of the sentence correctly, it
is incorrect
Option D- By is used to indicate means, nearness, time, order, according
to, manner or instrument, measure, in the name of. Since this
preposition does not fit correctly in the given sentence, it is
incorrect.