For this passage, the answers are in the details. A good strategy, like with most GMAT RC passages, is to create an outline in your head, encapsulating the main points of the passage:
OTs forage throughout lactation period (last up to 3 yrs)
PH. are larger and were thought to fast during their much shorter lactation periods.
Harbor seal, PH., challenges this traditional view, since it forages during lactation.
Possible theory: the harbor seal is much smaller than average PH, which have lots of blubber and can fast,
and is similar in size to OT, which don't have much fat stores.
Size is connected to foraging behavior and lactation.
Now to the questions

1) Otariids have a lactation period that is "4 months to 3 years"; phocines have one that "lasts from 4 and 50 days. Therefore, (A).
2) Harbor seals feed during lactation, which is uncommon in phocines (the second passage contrasts the grey seal, a phocine, which fasts throughout lactation). The point of the passage is to account for this behavior (the theory is that size plays a major role in lactation and foraging). The first paragraph mainly provides context for the difference between the two seal species in this regard.
(C) shifts the focus to current theories on harbor seals and how these theories are incorrect. The point of the passage is that the harbor seal represents an instance that should lead experts to reconsider their views on phocine as a whole, when it comes to foraging and lactation.
(E) goes too far since the passage never implies that the harbor seals belongs to the otariids, just that its behavior is atypical for a phocine, and therefore that not all phocine follow a pattern similar to that of the grey seal.
3) The last part of the first paragraph points to the idea that zoologists thought that all phocines fast throughout lactation, "fasting strategy in which mother phocids, having accumulated large energy stores before they arrive at breeding sites, fast throughout the entire lactation period". The "however" clues us into the fact that the behavior of the harbor seal changes this view.
4) The last part of the second paragraph arrives at the conclusion that the foraging behavior of the harbor seal is related to its size. To further back up this claim, the passage mentions the "ring seal", another small phocine that has similar foraging behavior during lactation as the harbor seal. The phrase "maternal strategy" represents foraging and lactation behavior.
Hope that helps