A fraction in which the numerator is greater than the denominator is an improper fraction.

$(a)\frac{12}{15}:$ Here $12<15\Rightarrow$Numerator $<$ Denominator.Hence it is not an improper fraction.

$(b)\frac{13}{17}:$ Here $13<17\Rightarrow$Numerator $<$ Denominator.Hence it is not an improper fraction.

$(c)\frac{16}{21}:$ Here $16<21\Rightarrow$Numerator $<$ Denominator.Hence it is not an improper fraction.

$(d)\frac{25}{11}:$ Here $25>11\Rightarrow$Numerator $>$ Denominator.Hence it is  an improper fraction.