Choose the correct adverb from the options given :
The train is _ _ _ _ _ _ _ _ crowded.
a) Today
b) Weekly
c) Always
d) Most highly

Hint:An adverb is a verb or phrase that modifies or qualifies an adjective, verb or another adverb or a word group expressing a relation of place, time, circumstance, manner, cause, degree, etc. For example: Gently, quite, then, there.

Complete answer:
In the given question, we need to fill in the blank with frequency adverbs. An adverb of frequency describes how often an action happens. There are six main adverbs of frequency that we use in English: always, usually, often, sometimes, rarely, never.

Now, let us examine all the given options and find out the correct answer:
Option ‘a’ is Today. It is an adverb, which is never used before an adjective. In the given sentence, ‘crowded' is an adjective, so we can’t use it in the given blank. Moreover, we need to fill the blank in with an adverb of frequency and not time. ‘Today’ is an adverb of time.
Option ‘b’ is Weekly. It is an adverb and it means ‘every week'. This is also an adverb of time and not frequency, hence it also cannot be the correct answer.
Option ‘c’ is Always. It is an adverb that means ‘all the time'. It is the adverb of frequency, clearly, as it is describing how often the train is crowded. This is the correct option.
Option ‘d’ is Most highly. This is grammatically not correct and “high” has its own superlative degree and “most” cannot be used with it.
As we have analyzed all the given options, we can easily figure out the correct answer. Here, the correct adverb for the blank would be – always. The complete sentence would be – The train is always crowded. So, the correct answer is “Option c”.

Note:
You must know how to differentiate between adverbs of frequency and adverbs of time. Adverbs of time tell a specific time period whereas adverbs of frequency tell how often in any task done.