Indefinite Article With A Plural Noun
The indefinite articles, a and an, are used to modify singular nouns. When using a plural noun, these two articles are unnecessary.
I have a books in my locker.
I have books in my locker.
I have a book in my locker.
It is also unnecessary to use an indefinite article if there is an adjective in front of a plural noun.
When he is in public, he does an embarrassing things.
When he is in public, he does embarrassing things.
When he is in public, he does an embarrassing thing.
When selecting courses, there are a many options to choose from.
A is not necessary in this sentence and should be removed. However, should you want to refer to the specific quantity of options, the sentence can be rephrased to use the definite article the:
You may select courses from the many options.