Question

even using the transit method, big planets are easier to spot than small ones, and to be sure that a flicker in brightness is caused by a planet rather than some property of the star itself, it must occur at regular and predictable
_____.
source: economist
intervals
pledges
decades
harvests

Explanation:

The text refers to the need for a regular and predictable occurrence related to the transit method for planet - spotting. "Intervals" means periods of time between events, which fits the context. "Pledges" are promises, "decades" are long periods of 10 years, and "harvests" are related to gathering crops, none of which make sense in this astronomical context.

Answer:

A. intervals