Answer: 4a^2 - 20a + 25

Step-by-step explanation:

A binomial ax^2+bx+c=0 is called perfect square trinomial

if b^2 = 4ac is satisfied.

For 49x^2 - 28x + 16

a = 49, b = -28 and c = 16,

(-28)^2=784

4times 49times 16 =3136

implies (-28)^2neq 4(49)(16)

Thus, 49x^2 - 28x + 16 is not a perfect square trinomial.

For 4a^2 - 20a + 25

a = 4, b = -20 and c = 25,

(-20)^2=400

4times 4times 25 =400

implies (-20)^2neq 4(4)(25)

Thus, 4a^2 - 20a + 25 is a perfect square trinomial.

For 25b^2 - 20b - 16

a = 25, b = -20 and c = -16,

(-20)^2=400

4times 25times 16 =-1600

implies (-20)^2neq 4(25)(16)

Thus, 25b^2 - 20b - 16 is not a perfect square trinomial.

For 16x^2 - 24x - 9

a = 16, b = -24 and c = -9,

(-24)^2=576

4times 16times -9 =-576

implies (-24)^2neq 4(16)(-9)

Thus,  16x^2 - 24x - 9 is not a perfect square trinomial.