'Knock on wood' is an idiom meaning to say for something not to happen, or maybe after it happened.
For example:
"We will win this basketball game, knock on wood."
In this case, he means that he doesn't want the opposite to happen; losing.
The correct answer is: Two to three times higher than among elderly, non-Hispanic whites. This is a bit of a sad fact but it is true, African American and Hispanic elders have a higher chance of being poor than non-Hispanic whites. There are many reasons for this phenomenon, but that is for a different discussion.
When something is most essential it means it is absolutely important or extremely necessary.
The answer is the last one. It starts in past tense by saying "was" but then changes to present tense by saying "starts".