。
She pointed out that the issue was a serious one.
Literal
She [topic-は] that issue [subject-が] serious is [nominalizer-こと] [object-を] pointed-out.
重大 ('serious, grave, weighty') is more formal than 大きい — it signals real consequence, the kind of word that shows up in news coverage of accidents, crimes, or major decisions. 指摘する ('to point out, indicate') is similarly bookish, common in academic writing and journalism. Note the structure ~であることを: to nominalize a non-verbal predicate (重大 is a na-adjective), Japanese uses である to attach copula-as-attribute, then nominalizes with こと, then marks the whole thing with を. The result reads almost like English's 'pointed out the fact that...'.