Select the correct direct form of the given sentence.

The Professor said that nobody could solve the problem.

The Professor said, ‘Nobody can solve the problem’.

The correct direct form of the given sentence is:

The Professor said, "Nobody could solve the problem."

The other options are incorrect for the following reasons:

• The Professor exclaimed, ‘Nobody can solve the problem?’ - The past tense verb "could" is not appropriate in this context, as the professor is talking about a general statement, not a specific event.
• The Professor said, ’Nobody could solve the problem’ - The quotation marks are missing.
• The Professor exclaimed, ‘Nobody could solve the problem’ - The past tense verb "could" is not appropriate in this context, and the exclamation point is not necessary.