The correct answer is: A. that
The word “that” is used to introduce a restrictive clause, which is a clause that limits the meaning of the noun or pronoun it modifies. In this case, the restrictive clause is “to be used widely.” This means that the sentence is saying that nuclear energy is so dangerous that it should not be used widely.
The other options are incorrect because they do not introduce restrictive clauses. The word “too” is used to compare two things, the word “such” is used to introduce a non-restrictive clause, and the word “so” is used to introduce a clause that expresses a result.
Here is a more detailed explanation of each option:
A. that
The word “that” is used to introduce a restrictive clause, which is a clause that limits the meaning of the noun or pronoun it modifies. In this case, the restrictive clause is “to be used widely.” This means that the sentence is saying that nuclear energy is so dangerous that it should not be used widely.B. too
The word “too” is used to compare two things. In this case, the sentence would be saying that nuclear energy is dangerous, and something else is also dangerous. However, the sentence does not compare nuclear energy to anything else, so this option is incorrect.C. such
The word “such” is used to introduce a non-restrictive clause, which is a clause that provides additional information about the noun or pronoun it modifies. In this case, the non-restrictive clause would be “to be used widely.” This means that the sentence would be saying that nuclear energy is dangerous, and it is also the case that nuclear energy is used widely. However, the sentence does not provide additional information about nuclear energy, so this option is incorrect.D. so
The word “so” is used to introduce a clause that expresses a result. In this case, the sentence would be saying that nuclear energy is dangerous, and the result of this is that it should not be used widely. However, the sentence does not express a result, so this option is incorrect.