Hi,

"practical" is the key term here. What was only in theory became a reality - hence, theoretical / abstract worry to practical problem.

From papayas in Hawaii to canola in Canada, the spread of pollen or seeds from genetically engineered plants is evolving from ________ scientific worry into a significant practical problem.

The meaning of the sentence is pretty straight: the OGM (organism genetic modified) is evolving from a concern, even though we do not have suffience study to asssert they are harmful for the environment and the human beings, from a theoretic standing point to a concrete one on the field.

of course, we do not know if this going through the initial stage and the actual one will resolve into more problem.

As matter of fact, from the theory to the concrete scenario we went

B and D are the right choices

From papayas in Hawaii to canola in Canada, the spread of pollen or seeds from genetically engineered plants is evolving from ________ scientific worry into a significant practical problem.

The blank calls for a word opposite in meaning to “practical”, thus, “theoretical” and “abstract”. Also the word “scientific” just after the blank justifies these words.