Try to evaluate the following using a suitable identity:
     (A) $35^2$
     (B) $65^2$
     (C) $85^2$
     (D) $105^2$

Use Śhrīdharāchārya's identity $a^2 = (a + b)(a - b) + b^2$ with $b = 5$ (or just note these are numbers ending in 5):

For any number ending in 5, let $k$ be the number formed by the digits before the final 5.
Then $(10k + 5)^2 = k(k + 1) \times 100 + 25$.

Thus, for any number ending in 5, the square can be obtained by writing 25 after the product $k(k + 1)$.