Senility is the quality of being senile (= showing poor mental ability because of old age). For example, " The hormone can damage brain cells and lead to premature senility." Hence, Option C is correct. Among the rest of the options, dotage means the period of life in which a person is old and weak, imbecility means stupid or inane behaviour, superannuation means a regular payment made into a fund by an employee towards a future pension. These options do not express the intended meaning, hence incorrect.