In Mass Effect 3,
Catalyst is the Intelligence created by the Leviathans to maintain peace between organics & synthetics. When that proved impossible, Catalyst chose to create the Reapers and start the cycles to prevent the inevitable conflict while preserving all advanced organic and synthetic life.
Shepard's arrival is evidence that the solution no longer works. A new solution is needed, and Catalyst knows that Crucible offers new options that Catalyst cannot achieve alone, but with Shepard's help it can. The three options are to Destroy, Control or Synthesis. Catalyst offers all three options to Shepard, yet at the same time, it already knows that Synthesis is the ideal solution: it is the final evolution of all life, organic and synthetic, and all of Catalyst's original objectives of preserving life and preventing organic-synthetic conflict can be achieved without the need of the cycles anymore.
The above is with reference to the below video major ending spoilers:
If Catalyst already believes that to be the ideal solution, why offer the others to Shepard at all and risk him not choosing the one Catalyst believes will achieve all of its objectives?
Looking for in-universe reasons not based on game mechanics, so "Catalyst needed him to have more War Assets to choose that" doesn't make sense in-universe.