The actual sentence -
A 2015 study by the U.S. Department of Energy found that currents
in U.S. waters were capable of generating up to 2000 terawatt-hours of energy each year, enough to meet half the country’s energy demand.
i.e.
Currents were capable of generating up to 2000 terawatt-hours of energy each year, enough to meet half the country’s energy demand.
What is enough to meet half the country’s energy demand?- Up to 2000 terawatt-hours of energy each year.
Thus, the phrase "enough to meet half the country’s energy demand" is a noun modifier that modifies the energy or precisely "up to 2000 terawatt-hours of
energy each year"
This construction has no issue.
A) enough to meet half the country’s energy demand.
Looks correct. Hold on.B)
which was enough to meet half the country’s energy demand.
"which" wrongly modifies "year"C)
and this was enough to meet half the country’s energy demand.
What is the antecedent of "this" ? Can't be -up to 2000 terawatt-hours of energy each year.
"This" is a demonstrative pronoun and can't stand alone to refer any noun. D)
and was enough to meet half the country’s energy demand.
S-V mismatch.The subject is plural "currents in U.S. waters ". Can't use was here.E) meeting half the country’s energy demand.
Currents were capable of generating ...., meeting half the country’s energy demand. Note: The participle modifier at the end (after comma) modifies the entire preceding clause and applies to the subject.
It can -
1. reflect the outcome of the action.
2.the way the action is performed. In this case, there is no action.
Note that the sentence says "Currents were capable of generating" and not that "Currents were generating".
Thus, the participle modifier after comma is not correct here.
IMO
A