Because the If-clause uses present tense -"is successful ", the following clause must use future tense -"will ". Only A and B remain.
If the development of a hardier variety of rice is successful, the damage toat least 4 million tons of rice crops a year will be prevented.
1) at least 4 million tons of rice crops a year will be
Correct.
2) 4 million or more than 4 million ons of rice crops a year will be
Awkward.
3) at least 4 million tons of rice crops a yearwere
4)
...
If the development of a hardier variety of rice is successful, the damage toat least 4 million tons of rice crops a year will be prevented.
1) at least 4 million tons of rice crops a year will be
Correct.
2) 4 million or more than 4 million ons of rice crops a year will be
Awkward.
3) at least 4 million tons of rice crops a yearwere
4)
...
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