It . . . . . . . . that, by 2050, world population . . . . . . . . twice as much as it is now.

Answer: Option B

Breaking Down the Sentence:
The sentence has two parts that need separate tenses:
Part 1: Describes the current belief ("It... that..."). This needs a present tense.
Part 2: Describes the future prediction about the world population ("by 2050, world population..."). This needs a future perfect tense (for an action completed by a specific time in the future).

Analyzing the Options:
Let's look at why the correct option is A and why the others are incorrect:
Option A: appears/will have risen
"Appears" is the present tense, correctly describing the current belief. "Will have risen" is the future perfect tense, correctly showing the population increase will be *completed* by 2050. This is the correct answer.

Option B: appeared/would have been rising
"Appeared" is past tense, which doesn't fit the present belief. "Would have been rising" implies something that was *in progress* but didn't finish by 2050. This is incorrect.

Option C: has appeared/will be rising
"Has appeared" is present perfect, suggesting the belief appeared at some unspecified point in the past and continues to the present. "Will be rising" is future continuous which implies an ongoing action. It doesn't show the completion of the rise by 2050. This is incorrect.

Option D: would appear/has risen
"Would appear" suggests a hypothetical belief, not a current one. "Has risen" is present perfect, showing an increase that happened, and is complete, before now. This is incorrect.

Option E: will appear/would rise
"Will appear" is future tense which doesn't describe the current belief. "Would rise" is conditional future, implying a hypothetical scenario. This is incorrect.