Find out the error according to the grammar and context of the given sentences. If there is no error, mark option 5,
i.e. No error. (Ignore the punctuation errors, if any.)
Understanding how particles such as (1)/ electrons travel vast distances in space (2)/ or how they acquire ultra-high energy (3)/ have been a long-standing puzzle in astrophysics .(4)/ No error (5)

The subject of the sentence is "Understanding", which is singular. Therefore, the verb should also be singular, i.e., "has been", not "have been."The confusion arises because "particles" and "electrons" are plural, but they are part of the object of the sentence. The subject of the sentence is "Understanding" (singular), so the correct verb form is "has been."Thus, the error is in (4) where "has been" should be used instead of "have been.""Understanding how particles such as electrons travel vast distances in space or how they acquire ultra-high energy has been a long-standing puzzle in astrophysics.