To answer this question, the best way is to use the dictionary. There are several online that can help you with these kinds of questions. If you are taking a test and a dictionary is not an option, there are a few things that might help, listed below.
When taking a multiple choice test, the best thing to do (which you probably already know) is to do away with words you are certain are incorrect responses.
In this question, "ocean" does not mean a large number. It is a vast area, but can only be measure by area, not by numbers. This would be the first word to dismiss. "Banner" usually is a cloth flag; banners in the past would fly at jousting tournaments—today they are present in stadiums, at state and government capitals, and with armies, etc. They are like flags. And based on the song, "banner" is a noun, described as "star spangled," seemingly having nothing to do with numbers. (This is using context clues...a very helpful skill to have...you may have it already, finding meaning for one word by studying the words that surround it.)
"Reposes" means to lounge or recline; rest. Examples from Dictionary.com are: ease, relaxation; or, lie calmly or quietly. This word has nothing to do with numbers, either. Using a root of a word (or in other cases, a prefix or suffix) can offer insight to what the word in question might mean. The root of the word "pose" can provide a clue that the word deals with the position of a body or thing: such as posing for a portrait. (See source listed below regarding "roots.")
One of the last two words in question provide the correct answer. "Ramparts" and "host" are words that will be the "deal breakers" here. If one does not know what the words mean, a guess is a possibility, but not an educated one...simply taking the 50-50 chance. However, a definition could be guessed at using context clues. In "The Star Spangled Banner," the lines regarding ramparts are as follows:
Whose broad stripes and bright stars thru the perilous fight,
O'er the ramparts we watched were so gallantly streaming?
A "rampart" is a fortified place—perhaps a fort or castle: anything protected against military assault. If you know the history of the song's origins, you might guess that a rampart (over which the flag was waving) was some kind of fort. This is not the answer you want.
"Host" would seem to be an answer easily discarded at first, but as many words have multiple meanings, it would be a mistake to do so. A "host" is generally someone who throws a party. However, if you have every heard of the "heavenly host," you will understand quickly that "host" in this context has nothing to do with a party. Another use of the word might be a "host of reasons not to go." It could also refer to a mighty army: "facing a heavily-armored host of the enemy."
"Host," therefore, means a large number.