54 is correct because a 9 KB item consumes three read capacity units for a strongly consistent read and 18 such reads per second require 54 RCUs.

DynamoDB charges one RCU per 4 KB or fraction thereof for strongly consistent reads and a 9 KB item therefore rounds up to three 4 KB chunks. Multiplying three RCUs per read by 18 reads per second yields 54 RCUs so that is the amount to provision.

72 is incorrect because it assumes four RCUs per 9 KB read which overestimates the rounded up chunk count.

27 is incorrect because that value reflects halving RCUs for eventual consistency and that rule does not apply to strongly consistent reads.

18 is incorrect because it ignores item size and treats each read as one RCU regardless of kilobytes.

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