To weaken the statement that the world's coal reserves will be exhausted by 2050, given the continued growth in demand, we need to find an option that presents a contrary or mitigating factor. Let's analyze the options provided:

A. There has been a slowdown in the rate of increase in world demand for coal over the last 5 years from 10% to 5%.
This option weakens the statement because a slower rate of increase in demand implies that the depletion of coal reserves may not occur as quickly as predicted. It suggests that demand growth is slowing down, which could extend the lifespan of coal reserves beyond 2050.

B. It has been known for many years that there are vast stocks of coal under Antarctica which have yet to be economically exploited.
This option weakens the statement because the existence of untapped coal reserves under Antarctica suggests that there are additional resources available. If these reserves become economically viable to exploit, it could significantly extend the lifespan of coal reserves beyond 2050.

C. Oil is being used increasingly in place of coal for many industrial and domestic uses.
This option weakens the statement because the increasing use of oil as a substitute for coal indicates a potential decrease in demand for coal. If industries and households are shifting to oil, it could reduce the rate of coal consumption and extend the lifespan of coal reserves.

D. As coal resources are depleted, more marginal supplies, which are more costly to produce and less efficient in use, are being mined.
This option supports the statement rather than weakening it. It suggests that as easily accessible and higher-quality coal resources are depleted, the remaining supplies become more expensive to produce and less efficient in use. This implies that the depletion of coal reserves may be accelerating.

E. Industries are increasingly using fossil fuels nowadays as new reserves of such energy have been discovered.
This option does not directly weaken the statement. While industries are using more fossil fuels, it does not specify whether coal specifically is being used more. Without information about the specific use of coal, it does not provide a clear contrary factor to the depletion of coal reserves.

Based on the analysis, the option that weakens the statement the most is option A: There has been a slowdown in the rate of increase in world demand for coal over the last 5 years from 10% to 5%. It suggests a potential decrease in demand growth, which could extend the lifespan of coal reserves beyond 2050.