What could be the MOST problematic for this TAS?

A web application was released into production one year ago, it has regular release which follow a V-model lifecycle and testing is well-established and fully integration into the development lifecycle. You have been asked to implement a TAS for the regression test suite. The regression tests have been developed via the GUI and are expected to be run at least four times a month, for each planned release, for the whole operation solution life of the system (six years). Each screen of the GUI uses several third-party controls which are not compatible with the existing automation solutions. The environment for the automation will be stable, fully controllable and separated from other environments (development, staging, production).

What could be the MOST problematic for this TAS?
A . Maturity of the test process
B. Complexity to automate
C. Frequency of use
D. Sustainability of the automated environment

Answer: B