Choice A is the best answer because a comma after the word “hydrant” separates the phrase “a pigeon pecking for crumbs around a fire hydrant” from the phrase “an old man tending to a baby outside a doorway.” A comma is also consistent with the punctuation choice made to separate the first two phrases in the asyndetic series following the colon in the sentence.

Choice B is not the best answer because a dash is not a conventional choice for punctuating items in a series.

Choice C is not the best answer because although a colon can be used to introduce a series, it is not an conventional choice for separating items within a series.

Choice D is not the best answer because it fuses together two items in the series. Separating the phrases “a pigeon pecking for crumbs around a fire hydrant” and “an old man tending to a baby outside a doorway” requires punctuation and/or a coordinating conjunction.