Am irregular verb means that the different forms of the verb will differ and will not follow a regular pattern,
The pattern here should be:
I VERB, I VERB-ed, I will VERB
the first three options follow this pattern, so they are regular,
the correct answer is D. it is irregular: the past tense form is irregular.
By birth? i think? do you mean like how did they get in a certain caste? because then it would be by birth. but if you mean like what were the castes, i forget.
The poor standing of Leopold the 2nd is most probably attributed to the role he had in the extermination of huge amounts of people in Belgian colonies at the time. This wasn't known at the time, but today we known that Leopold the 2nd had ordered a genocide over the Congonese people.