Until now, American cell-phone manufacturers enjoyed a large

I agree it is (E).
Stem: Chinese companies now have greater shares of the market. Therefore Amercian companies will see a diminishing shar of profits from China.

(A) Very few cell phones manufactured in China will be exported for sale in the United States.
Do not attack the argument. No exports will not help American companies' profit from China.

(B) There are still more cell phone users in the United States than China, despite the latter's greater population.
Yes, but American companies' profit in China may still be decreasing.

(C) American companies still claim large shares of sales in China of other electronic devices such as computers.
It may be true, yet it does not help American companies' profit in cell phone sales in China.

(D) Many American cell phones are considered to be of superior quality to those produced by Chinese manufacturers.
Again, it may be true. However the fact is that American cell phone companies' market share in China has been decreasing when Chinese companies' claiming larger and larger market share, as claimed in the question stem. In other words the good quality has not helped with the declination of the market share, thus cannot help with the declination of the profit from China.

(E) Cell phones manufactured by Chinese companies use many imported electronic components.
The apparent increase in Chinese companies' market share is less harmful to American companies if this is true, as American companies simply shift their focus from whole product to electronic components and can still maintain their profitability in China.

Let me add my 2 cents:
(D) assumes Chinese customers prefer superior quality cell phones.
(E) assumes imported electronic items come only from those US companies which currently are in chinese market.

Probably, it can also be argued that the Superior quality doesn't mean more sales to American companies.

Your problem with this is that this is not a point for any further arguing. In the stem it clearly stated that the fact is Amercian companies' market share has been and is decreasing. The superior quality argument will not help because the fact is already given. The link you need to attack is why decreasing market share doesn't necessarily mean decreasing profit margin. You cannot attack the given fact and hope to change it. It is already established.

Your problem with this is that this is not a point for any further arguing. In the stem it clearly stated that the fact is Amercian companies' market share has been and is decreasing. The superior quality argument will not help because the fact is already given. The link you need to attack is why decreasing market share doesn't necessarily mean decreasing profit margin. You cannot attack the given fact and hope to change it. It is already established.