Option 1 -> If TR is a straight line, MR is constant, not falling.
Option 2 -> If TR is a straight line, MR is constant, not increasing.
Option 3 -> When TR is an upward sloping straight line through the origin, TR = P × Q where P is constant. Thus MR = dTR/dQ = P and AR = TR/Q = P. Therefore, MR = AR.
Option 4 -> MR equals AR, it is not greater than AR.
Hence, Option 3: Marginal revenue is always equal to average revenue -> When the total revenue curve is an upward sloping straight line (typically passing through the origin), it indicates a perfectly competitive market where the firm is a price taker. Here, TR = P × Q where price (P) remains constant. The slope of this straight line gives us MR, which equals P. Similarly, AR = TR/Q = P. Since both MR and AR equal the constant price P, they are always equal to each other. This is a characteristic feature of perfect competition. -> correct