Which sentence uses the word benign correctly? my mother's cookies were fresh out of the oven and totally benign.

D. The stingray had had its stinger removed and was completely benign.

Explanation:

In the incomplete set of options given in the question, what we can do first is to focus on the meaning of the word "benign" first. Benign means something or someone who is calm, gentle, not harmful. It is the opposite of violent, rough etc. So, taking this meaning into consideration, we can now try to incorporate this word to the way they are used in the sentences in the option.

While the first option use it as-

Don't be so benign as to forget your homework again

This usage is wrong.

Again, be it the second or the third option which reads as

My mother's cookies were fresh out of the oven and totally benign.

I was benign as a child, but have learned to speak up for myself as an adult.

In both sentences, the usage of this word is incorrect and makes their sentence construction seem awkward.

Whereas, the use of this word in the last option-

The stingray had had its stinger removed and was completely benign

is correct for it signifies that the stingray had become calm and harmless after having its stinger removed.

Thus, the correct answer is option D.

coin toss of the tosses to the toss