@asutosh
As someone has pointed out, there isn't much to learn from this question. The first questionable thing is that a budget is normally a yearly thing. Therefore, there is no need to say, annually and dollars a year, a rampant redundancy. Again, this says " $220 a year " implying that the figure is a fixed amount for all the budgets that might come in the future also. A budget changes every year and in some countries, the budgets are reviewed periodically and revised within the same year.
Look at another goof-up. This says a budget of "$220 billion dollars". This is not the way the currencies are expressed. One normally says that it is "$220B" or ' $220bn" or 220 billion dollars without the $ symbol in front.
Further, there is no such thing as a common GDP for many nations. Every country has its own GDP and when referring to several nations, we must actually say GDPs.
I am afraid that there isn't anything to learn from this question.
However, that aside let's now try to go about. I don't agree with the OA. Here $220B is a quantum of money. When a quantum is expressed especially in the case of currencies, distances and time measurements, we can comfortably say.
1. Rs 5000 a shirt is more than what an executive can afford to pay.
2. The distance of 12000 kilometers is more than what some birds can travel at a stretch.
3. The time of 2 to 3 hours taken by some aircraft is more than what a jet normally takes.
Please note in all the above cases, the measurements are themselves in plural but they all take 'more than' rather than 'greater than'.
But if you are meaning to solve this question through grammar, I may say, there is no point in taking even A as the least objectionable. had it not been for the distinction between more than, and greater than, one might grudgingly take A.
Quote:
The United States Department of Defense is the single largest bureaucratic institution in the United States, with an annual budget of $220 billion dollars a year, greater than the Gross Domestic Product of several nations on earth.
(A) greater than ---
You may take this if you want. (B) greater than that of --- '
That' doesn't work here. 'That' stands for the budget and therefore there is no such thing as the budget of the GDPs of other nation. (C) more than that of ---
the same problem as in B. (D) more than in --
the preposition 'in' does not fit in anywhere. (E) which is more than --
...'which' has no reference. It points neither to the year nor to the $220 billion dollars, a plural noun.