A muff can't add a new force to an ungrounded fumble = the fumble was grounded.
B forced the fumble back thru it's own end zone = safety.
Test answer = FALSE.
IMHO, questions such as these cause discussions. Discussions sometimes help in getting a better understanding of the rules. Having a better understanding of the rules makes for a better official.

IMHO, if the ball is bouncing toward the goal line don't try to guess if the muff added a new force that enabled the ball to reach the end zone...IT DIDN'T. If the ball is at rest OR bouncing away from the goal line...IT DID.

Many years ago ,when OUR New England Patriots

were

THOSE Boston Patriots , the following occurred:
(1) Pats at their 15;
(2) Pats fumble ;
(3) Bills player tries to field airborne fumble;
(4) ball deflects off Bilkl's player's knee;
(5) ball bounces back thru Pat's EZ;
(6) I yelled to my buddies : "WOW, that's going to be a touchback

!!";
(7)the white hat announced to the frenzied masses : "Because the fumble had not touched the ground, the Patriots were responsible for the ball leaving the end zone and a safety is scored.
(8) I had another beer

.
While the above play might seem unfair, it falls into the same rule as a blocked punt bouncing back thru K's end zone. We all would consider that fair.