If the fuse in the plug of a microwave oven is replaced.....

The answer is D
Notice the use of the word [may].

A. The microwave oven will not operate.
ans. almost, YES, subject to size of copper strip. Oven approx. 1000w, BS1361 fuse 10A is to be suitable. HKD 2 or 3 dollars /each

B. The electric current flowing through the microwave oven will be doubled up.
ans. remained unchange; protection term, not operation..

C. The efficiency of the microwave oven becomes higher.
ans. No. (remained unchange..)

D. A large current may pass through the oven.
ans. No. (same current pass through the fuse or strip)