The correct option is B stronger than
The sentence should be in the comparative degree of comparison. When a comparison is made between a noun that is higher in quality compared to the other noun, then the sentence is of the comparative degree. Here, the comparison is made between 'Mary' who has more strength, 'stronger' than 'Ellen'. Option C is of the comparative degree; 'stronger' is the comparative of 'strong' and 'than' is the link between the nouns, hence it's the correct answer. Option A is incorrect because the word alone shows no degree of comparison. Option B is incorrect because its degree of comparison is positive (as strong as). Option D is incorrect because it's of the superlative degree (strongest).