Join the sentences using a suitable connector from those given in the options.
He failed the exam. He had not studied.
(This question has multiple correct options)
a. For
b. However
c. Since
d. As

Hint: A conjunction is used to join two separate sentences and form a single sentence.

Complete answer:
From the hint given to us, we know that a conjunction is used to join two separate sentences and form a single sentence. Thus, we can infer that we have to use a suitable conjunction to join the given two sentences.
a. For is a preposition and is used to convey an indication or a purpose. We observe that it does not apply to the given context and hence, option A is not the correct answer.
b. However, is a linker which is used to introduce a statement that contrasts or contradicts the previous sentence. It does not merge two sentences to form one sentence.We observe that it does not apply to the given context and hence, option B is not the correct answer.
c. Since is a conjunction which used to express a cause or indicate a time period. We observe that it is suitable to the given context. However, it is ambiguous in nature due to its variable meanings. Hence, option C is not the correct answer.
d. As is a conjunction used to introduce a cause. We observe that it is suitable for the given context and hence, we can conclude that option D is the correct answer.

Note: As is more formal than SInce. However, in place of these two conjunctions, ‘because’ can also be used.