He thinks about the welfare of the citizens most seriously.
He is someone who considers the welfare of the citizens seriously the most.
"He thinks about the welfare of the citizens"=「彼は国民の幸福な生活のことを考える」
"most seriously"＝「最も真剣に」
あるいは、
"He is someone"＝「彼は～という人間だ」
"who considers the welfare of the citizens seriously"＝「真剣に国民の幸福な生活について考慮する」
"the most"＝「最も」
アメリカ合衆国 
