Fill in the blank with suitable conjunction-
__________ I were a bird, I would fly high.
a) Since
b) For
c) Because
d) If

Hint: Read the given phrase/sentence carefully. Try to make out the meaning of it and find the subject or the predicate or the main object of the sentence.

Complete answer:
 Conjunctions are the words that are used to connect clauses or sentences or to coordinate words in the same clause, for example: and, but, if, etc. The given sentence conveys that if the speaker were a bird, he/she would fly high.
Now, let us examine all the given options to find out the correct option-
Option ‘a’ is Since. It is a from of time in the past until the time under consideration, typically the present; for the reason that, because. It cannot be used in the given sentence.
Option ‘b’ is For. It is used to specify a period of time. It cannot be used in the given sentence. So, we will eliminate this option.
Option ‘c’ is Because. It means for the reason that; since. We cannot use it in the given sentence. So, we will eliminate this option.
Option ‘d’ is If. It is a conjunction which means on the condition or supposition in the event that. It can be used in the given blank and is the correct answer. Hence, option ‘d’ is the correct option.

Note:
 All the other options are plausible distractions. The words ‘since’ and ‘because' have the same meanings and can be used in place of each other.