A trailer jackknife is most likely to happen when:

The trailer is empty or lightly loaded.. Section 6.1: Empty trailers have less traction and are more likely to lock up and jackknife.

ANSWER BREAKDOWN

A. Incorrect. "The trailer is full." is not correct. Empty trailers have less traction and are more likely to lock up and jackknife.
B. Correct. Empty trailers have less traction and are more likely to lock up and jackknife.
C. Incorrect. "You are driving uphill." is not correct. Empty trailers have less traction and are more likely to lock up and jackknife.
D. Incorrect. "You are parked." is not correct. Empty trailers have less traction and are more likely to lock up and jackknife.

Un plegado en tijera del remolque es mas probable cuando:

AThe trailer is full.

BThe trailer is empty or lightly loaded.

CYou are driving uphill.

DYou are parked.