Find the next term in the A.P. 

$a, 2a, 3a, 4a, \dots$

Here,

$a_2 – a_1 = 2a – a = a$

$a_3 – a_2 = 3a - 2a = a$

$a_4 – a_3 = 4a - 3a = a$

The common difference $\left(d\right)$ comes out to be $a$,  

Hence, the next three terms can be found using:

$a_{n }= a_{n-1}+d$

$a_5 = 4a+a = 5a$

$a_6 = 5a+a = 6a$

$a_7 = 6a+a = 7a$

Therefore, $5a, 6a, 7a$ is the correct option.