Explanation
References: SSL Proxy Overview Configuring Certificate Authority Profiles Configuring a Root CA Certificate

Explanation
The cSRX is a containerized version of the SRX Series device that runs on Linux platforms. The cSRX has the following benefits for deploying in a virtual environment:
The cSRX supports Layer 2 and Layer 3 deployments, which means it can operate as a transparent or a routed firewall. The cSRX can also support multiple routing instances and virtual routers2 The cSRX supports firewall, NAT, IPS, and UTM services, which means it can provide comprehensive security features for protecting the virtual network. The cSRX can also support application identification, user firewall, and VPN services2 The cSRX default configuration does not contain three default zones: trust, untrust, and management. The cSRX default configuration contains only one zone: junos-host. The cSRX does not have low memory requirements. The cSRX requires at least 2 GB of memory and 2 CPU cores to run2 References:
1: Juniper Security Specialist (JNCIS-SEC) (JN0-334) | Study Guide 3
2: Junos OS Security Configuration Guide | cSRX Overview 4

Explanation
This is because the error message indicates that the client device does not trust the certificate authority that issued the certificate for the external server. Therefore, you need to import the root CA certificate from the SRX Series device to the client device and add it to the trusted certificate store. This will allow the client device to verify the identity of the SRX Series device and accept the certificates it generates for the external servers3.
The other options are not correct because:
A). Load a known good, but expired. CA certificate onto the SRX Series device. This will not solve the problem because the client device will still reject the certificates from the SRX Series device if they are signed by an expired CA certificate. The expiration date of the CA certificate is one of the factors that the client device checks when validating the certificate2.
B). Install a new SRX Series device to act as the client proxy. This will not solve the problem because the new SRX Series device will still need to have a root CA certificate configured and imported to the client devices. The problem is not with the SRX Series device itself, but with the trust relationship between the SRX Series device and the client devices2.
C). Reboot the SRX Series device. This will not solve the problem because rebooting the SRX Series device will not change the configuration or the certificates on the SRX Series device or the client devices. The problem is not caused by a temporary glitch or a malfunction, but by a mismatch between the certificates on the SRX Series device and the client devices2.
I hope this helps you understand the problem and the solution better. If you want to learn more about SSL client protection proxy, you can check out the following references:
SSL Proxy Overview
Configuring SSL Forward Proxy
Configuring a Root CA Certificate
SSL Proxy - Client Protection