Fill in the blank space for the given series with the correct alternative.
$2, 15, 41, 80, (\ldots)$

Find the missing letters for the given series from the given alternatives:
$b....ac....cc....cb...ab....ac$

The positions of how many digits in the number $53147926$ will remain unchanged after the digits within the number are rearranged in descending order?

These questions are based on the following arrangement. Study it carefully and answer the questions.
$J, \star, R, 3, P, L, 2, \#, I, N, 7, O, C, @, K, 5, D, =, M, \$, 6, B, >, A, Q, 4$
What will come in place of the question mark in the following series based on the above arrangement?
$PRJ, \# L3, 7I2, @ON, ?$

Choose the correct alternative to complete the series: $...aba...cabc...dcba...bab...a$

Choose the correct alternative for the missing term in the series: $ejo, tyd, ins, xch, ?$