a3= 1010101 is a composite number with a factor '101'. If we consider the option d), it states that 10^(3+1) +1 is divisible by 11 which is not true. Hence option d) is incorrect.
Option a) also gets eliminated as 3-2=1, is not divisible by 3.
a4 is prime so option c) gets eliminated. Hence option b) is the correct answer.
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