C. It erodes away the material under it until the arch crashes down as well
B) to assure himself of its truth
The options given have you take a closer look at the words it's and himself. Option C changes "himself" to "hisself". "Hisself" is never correct. This means we have to decide if the correct answer is "its" or "it's". "Its", without an apostrophe, is the possessive form showing ownership. "It's", with an apostrophe, is a contraction meaning "it is". Let's plug them into the sentence "of it is truth" does not make any sense. Can "it" own truth? Yes, so "its" is the correct answer.
Well depends on what year it is but in the early ages it was for land, might need more details though
The British Empire. Post-war USA The coolness factor