la seule chose dont il se

the only thing he … about

→ le
phrase Rare #50,025
the only thing he … about This is an incomplete phrase; the verb that follows determines what he does about the thing (for example, se soucier de means “to care about”).
FR: La seule chose dont il se soucie, c'est son travail.
EN: The only thing he cares about is his work.

La seule chose dont il se soucie, c'est le fric.

The only thing he cares about is money.

La seule chose dont il se préoccupe, c'est le fric.

The only thing he cares about is money.

La seule chose dont il se soucie, c'est l'argent.

The only thing he cares about is money.

La seule chose dont il se préoccupe, c'est l'argent.

The only thing he cares about is money.