Step-by-step explanation:

(1): "b2"   was replaced by   "b^2".  1 more similar replacement(s).

((a2) - 10ab) + 52b2

2.1    Factoring    a2 - 10ab + 25b2 

Try to factor this multi-variable trinomial using trial and error 

 Found a factorization  :  (a - 5b)•(a - 5b)

 2.2    a2  -10ab  +25b2  is a perfect square 

 It factors into  (a-5b)•(a-5b)

which is another way of writing  (a-5b)2

FINAL RESULT:

(a - 5b)2

so the answer is B.