Answer:The geometric series is: 2+14+98+686Step-by-step explanation:The geometric series is a series in which each term of the series is a constant multiple of the preceding term of the series.a)6+13+20+27This is not a geometric series but is a arithmetic series.Since, each term is increased from the preceding term by 7.Since, 6+7=1313+7=2020+7=27b)7+21+35+45This is not a geometric series.Since 7×3=21But 21×3=63 But the next term that is given is: 28 and not 63c)14+21+28+35This is a arithmetic sequence.Since14+7=2121+7=28and 28+7=35d)2+14+98+686This is a geometric sequence.Since,. we get a common ratio i.e. a term which when multiplied to each term of sequence gives the next term of the series.i.e. 2×7=1414×7=9898×7=686